This answer was provided by user Hassan in the comments on the original question. I’m only posting it as an answer to give it better visibility.
Basically, all you need to do is add server.error.include-message=always to your application.properties file, and now your message field should be populated.
This behavior was changed in Spring Boot 2.3 which you can read about here: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.3-Release-Notes#changes-to-the-default-error-pages-content