As documented, the generated sources for type converter loaders will be created after compiling. This means that the type converter loaders will not be available to unit tests, etc.
This will go unnoticed if you (or your IDE) previously compiled the source code, thus run the generator. But in a CI environment, the problem is obvious, particularly if you have unit tests that use type converters.
You can also test with `rm -rf src/generated; mvn install`.
The solution is to change the phase from `process-classes` to `pre-compile`.