diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
index 7a95434248a5..d16da5a634a9 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
@@ -726,8 +726,12 @@ public Schema normalizeSchema(Schema schema, Set<Schema> visitedSchemas) {
             normalizeSchema(result.getItems(), visitedSchemas);
             return result;
         } else if (schema.getAdditionalProperties() instanceof Schema) { // map
-            normalizeMapSchema(schema);
-            normalizeSchema((Schema) schema.getAdditionalProperties(), visitedSchemas);
+            if(!ModelUtils.isModelWithPropertiesOnly(schema)) {
+                normalizeMapSchema(schema);
+                normalizeSchema((Schema) schema.getAdditionalProperties(), visitedSchemas);
+            } else {
+                normalizeProperties(schema.getProperties(), visitedSchemas);
+            }
         } else if (ModelUtils.isOneOf(schema)) { // oneOf
             return normalizeOneOf(schema, visitedSchemas);
         } else if (ModelUtils.isAnyOf(schema)) { // anyOf
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java
index ed4287ab0fc0..55be06da8003 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java
@@ -40,6 +40,7 @@
 import org.junit.jupiter.api.Assertions;
 import org.openapitools.codegen.config.CodegenConfigurator;
 import org.openapitools.codegen.config.GlobalSettings;
+import org.openapitools.codegen.java.assertions.JavaFileAssert;
 import org.openapitools.codegen.languages.SpringCodegen;
 import org.openapitools.codegen.model.ModelMap;
 import org.openapitools.codegen.model.ModelsMap;
@@ -55,6 +56,7 @@
 import java.nio.file.Files;
 import java.util.*;
 import java.util.concurrent.*;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 import static org.assertj.core.api.Assertions.assertThat;
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java
index 1d41104f8dc0..d7fb7d51ef3d 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java
@@ -939,6 +939,11 @@ public void testOpenAPINormalizerSimplifyOneOfAnyOf31Spec() {
         assertEquals(schema22.getAnyOf(), null);
         assertEquals(schema22.getTypes(), Set.of("string"));
         assertEquals(schema22.getEnum().size(), 2);
+
+        Schema schema23 = openAPI.getComponents().getSchemas().get("AnyOfNullableAdditionalPropertiesTest");
+        assertEquals(((Schema) schema23.getProperties().get("str")).getAnyOf(), null);
+        assertTrue(((Schema) schema23.getProperties().get("str")).getNullable());
+        assertEquals(schema22.getTypes(), Set.of("string"));
     }
 
     @Test
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
index 0b84b410fae0..ed0388fb3e5a 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
@@ -67,6 +67,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.entry;
 import static org.assertj.core.api.InstanceOfAssertFactories.FILE;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.openapitools.codegen.CodegenConstants.*;
 import static org.openapitools.codegen.TestUtils.*;
 import static org.openapitools.codegen.languages.JavaClientCodegen.*;
@@ -3833,6 +3834,30 @@ public void queryParameterJsonSerialization(String library) {
         );
     }
 
+    @Test(description = "Issue #20213")
+    public void givenModelHasFalseAdditionalPropertiesAndPropertyHasNullAsAnyOfTypeThenModelIsCorrect() throws Exception {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setInputSpec("src/test/resources/3_1/issue_20213.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator defaultGenerator = new DefaultGenerator();
+        Map<String, File> fileMap = defaultGenerator.opts(clientOptInput).generate()
+                .stream().collect(Collectors.toMap(File::getName, Function.identity()));
+
+        JavaFileAssert.assertThat(fileMap.get("SampleObjectWithAdditionalFalse.java"))
+                .assertProperty("someString")
+                .withType("String")
+                .assertPropertyAnnotations()
+                .containsWithName("javax.annotation.Nullable");
+        assertFalse(fileMap.containsKey("SampleObjectWithAdditionalFalseSomeString.java"));
+
+    }
+
     @DataProvider(name = "springClients")
     public static Object[] springClients() {
         return new Object[]{RESTCLIENT, WEBCLIENT};
diff --git a/modules/openapi-generator/src/test/resources/3_1/issue_20213.yaml b/modules/openapi-generator/src/test/resources/3_1/issue_20213.yaml
new file mode 100644
index 000000000000..2726053d0a8b
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_1/issue_20213.yaml
@@ -0,0 +1,30 @@
+# filename: nullable-properties-with-additional-properties-false.yaml
+openapi: 3.1.0
+info:
+  title: ""
+  version: 1.0.0
+components:
+  schemas:
+    # For reference, an object without additionalProperties: false, but with nullable properties
+    SampleObject:
+      properties:
+        someString:
+          anyOf:
+            - type: string
+            - type: 'null'
+
+    # For reference, an object with additionalProperties: false, but without any nullable properties
+    ReferenceObject:
+      additionalProperties: false
+      properties:
+        someString:
+          type: string
+
+    # The broken case: an object with additionalProperties: false and nullable properties
+    SampleObjectWithAdditionalFalse:
+      additionalProperties: false
+      properties:
+        someString:
+          anyOf:
+            - type: string
+            - type: 'null'
\ No newline at end of file
diff --git a/modules/openapi-generator/src/test/resources/3_1/simplifyOneOfAnyOf_test.yaml b/modules/openapi-generator/src/test/resources/3_1/simplifyOneOfAnyOf_test.yaml
index 588abf3e84c2..cbc387d8c514 100644
--- a/modules/openapi-generator/src/test/resources/3_1/simplifyOneOfAnyOf_test.yaml
+++ b/modules/openapi-generator/src/test/resources/3_1/simplifyOneOfAnyOf_test.yaml
@@ -137,4 +137,14 @@ components:
     OneOfNullAndRef3:
       oneOf:
         - $ref: '#/components/schemas/Parent'
-        - type: "null"
\ No newline at end of file
+        - type: "null"
+    AnyOfNullableAdditionalPropertiesTest:
+      description: to test anyOf with additional properties
+      additionalProperties: false
+      properties:
+        str:
+          anyOf:
+            - type: string
+            - type: 'null'
+            - type: null
+            - $ref: null
\ No newline at end of file