It started after updating spring-doc dependency in some project. [Here is springdoc-openapi issue I created](https://github.com/springdoc/springdoc-openapi/issues/3016#issuecomment-2966372532). [It contains minimal reproducible example project with springboot - kotlin](https://github.com/damiankaplon/springdoc-kotlin-enum-as-ref). I created [a draft PR](https://github.com/swagger-api/swagger-core/pull/4931) which contains failing tests. The test fails only for open api version 3.1. However the test is passing for 3.0