Please see the following page for further information regarding protobuf versioning:

https://protobuf.dev/support/version-support/

As the protobuf project contains implementations for many languages each language can increment the major version independently.

This means for protobuf-java the tag for 4.28.3 is actually https://github.com/protocolbuffers/protobuf/releases/tag/v28.3 the Java version being visible at https://github.com/protocolbuffers/protobuf/blob/v28.3/java/pom.xml#L7

The full diff from v25.5 to v28.3 can be seen at:

https://github.com/protocolbuffers/protobuf/compare/v28.3...v25.5

The two attached files are the logs and diff for the changes to the "java" directory only.