Historically we always allowed to project on fields we don't know about: we'll simply assume they exist, and return them as-is.

When this decision was made - a long time ago - Lucene only had Strings so there was no ambiguity about how it should be decoded; nowadays I think we can consider not supporting this same level of flexibility for the ES integration. We might also want to discuss dropping this for the Lucene backend, although that would have to wait for 6.0.