Are you using the EJB @Transactional by the way? I just played with a minimal sample and for some reason can add it to my codebase and it compiles but then at runtime the class not being detected and thus the transaction attribute lookup failing
Nevermind, I got it. The custom Spring Data JPA TransactionAttributeSource doesn't consider JTA 2.1's @Transactional which you apparently are using. Spring Boots starter for JPA pulls that one in, which makes it available for import in the code but then it's ultimately Spring Data JPA not considering it due to a special TransactionAttributeSource that's not aware of JTA 2.1's @Transactional yet.