The @JsonTest annotation automatically includes beans of type com.fasterxml.jackson.databind.Module.
So you can test that you've configured Jackson correctly by using a JsonTest, and it will automatically pick up your beans of type Module.
But as soon as you use an @WebMvcTest that relies on this Module bean, it fails because @WebMvcTest does not include beans of type com.fasterxml.jackson.databind.Module (although it does include beans annotated with @JsonComponent, just like @JsonTest).
This is easy enough to workaround, but it's surprising and inconsistent, IMHO.