diff --git a/x-pack/solutions/security/plugins/security_solution/server/lib/detection_engine/prebuilt_rules/logic/diff/calculation/calculate_rule_fields_diff.ts b/x-pack/solutions/security/plugins/security_solution/server/lib/detection_engine/prebuilt_rules/logic/diff/calculation/calculate_rule_fields_diff.ts
index 41625af391bd2..0f2544ca99f5c 100644
--- a/x-pack/solutions/security/plugins/security_solution/server/lib/detection_engine/prebuilt_rules/logic/diff/calculation/calculate_rule_fields_diff.ts
+++ b/x-pack/solutions/security/plugins/security_solution/server/lib/detection_engine/prebuilt_rules/logic/diff/calculation/calculate_rule_fields_diff.ts
@@ -219,7 +219,7 @@ const commonFieldsDiffAlgorithms: FieldsDiffAlgorithmsFor<DiffableCommonFields>
   version: forceTargetVersionDiffAlgorithm,
   name: singleLineStringDiffAlgorithm,
   tags: createScalarArrayDiffAlgorithm({
-    missingBaseVersionStrategy: ScalarArrayDiffMissingBaseVersionStrategy.Merge,
+    missingBaseVersionStrategy: ScalarArrayDiffMissingBaseVersionStrategy.UseTarget,
   }),
   description: multiLineStringDiffAlgorithm,
   severity: singleLineStringDiffAlgorithm,
diff --git a/x-pack/test/security_solution_api_integration/test_suites/detections_response/rules_management/prebuilt_rules/prebuilt_rule_customization/customization_enabled/diffable_rule_fields/common_fields/tags.ts b/x-pack/test/security_solution_api_integration/test_suites/detections_response/rules_management/prebuilt_rules/prebuilt_rule_customization/customization_enabled/diffable_rule_fields/common_fields/tags.ts
index ecaa093a7f359..1f67e55692fa1 100644
--- a/x-pack/test/security_solution_api_integration/test_suites/detections_response/rules_management/prebuilt_rules/prebuilt_rule_customization/customization_enabled/diffable_rule_fields/common_fields/tags.ts
+++ b/x-pack/test/security_solution_api_integration/test_suites/detections_response/rules_management/prebuilt_rules/prebuilt_rule_customization/customization_enabled/diffable_rule_fields/common_fields/tags.ts
@@ -297,11 +297,11 @@ export function tagsField({ getService }: FtrProviderContext): void {
             ruleUpgradeAssets,
             diffableRuleFieldName: 'tags',
             expectedDiffOutcome: ThreeWayDiffOutcome.MissingBaseCanUpdate,
-            isMergableField: true,
+            isMergableField: false,
             expectedFieldDiffValues: {
               current: ['tagB'],
               target: ['tagC'],
-              merged: ['tagB', 'tagC'],
+              merged: ['tagC'],
             },
           },
           getService