As far as I can tell, we never take into account the fact that converters are ignored when validating the compatibility of fields when targeting multiple indexes.
Our tests pass, but just because I made a poor job of designing the converters: they happen to always be compatible...

We should:

1. Change org.hibernate.search.integrationtest.backend.tck.testsupport.util.ValueWrapper#toIndexFieldConverter and org.hibernate.search.integrationtest.backend.tck.testsupport.util.ValueWrapper#fromIndexFieldConverter to pass some parameter that will ensure converters are only deemed compatible when we need them to be

2. Change org.hibernate.search.backend.elasticsearch.search.projection.impl.ElasticsearchSearchProjectionBuilderFactory#field and similar so that they do not fail if we pass ProjectionConverter.DISABLED and the projection converters are not compatible.