It seems the compatibility check we do in org.hibernate.search.backend.lucene.types.projection.impl.StandardFieldProjectionBuilderFactory#isDslCompatibleWith (for example) only calls org.hibernate.search.backend.lucene.types.converter.impl.LuceneFieldConverter#isDslCompatibleWith, which calls org.hibernate.search.engine.backend.document.spi.UserIndexFieldConverter#isDslCompatibleWith, which checks the compatibility of the DSL converter (which by the way, doesn't matter for projections), and more importantly does not check compatibility of the projection converter.

We should have a look at that, add relevant tests and fix the issue.