You discovered the root cause: your old Hibernate Validator version supports org.hibernate.validator.constraints.NotBlank but not the javax.validation.constraints.NotBlank properly.

Upgrading Hibernate Validator to 6.x fixes the problem, but you're stuck with 5.1.0 due to project constraints.

So your version of Hibernate Validator simply does not recognize or enforce the newer javax.validation.constraints.NotBlank annotation.

suggested solutions:

• The cleanest approach without upgrading is to replace all usages of javax.validation.constraints.NotBlank with org.hibernate.validator.constraints.NotBlank in your code.
• If you must stick to javax annotations for some reason, fallback to @NotNull + manual empty-string checks, or implement a custom validator and use it explicitly.