EF supports "shadow properties" which allow you to specify parts of the EF metadata that round-trip but are not mapped to a POCO property.

With us relying on the V3 provider which is POCO property based we can't currently pass these through (although we could materialize them due to the BsonDocument serializer).

This isn't so much of a problem for us vs Cosmos etc. as we just updated the changed properties and do not replace the entire document so fields are not lost.

I recommend we indicate in the docs these are not supported and then monitor customer feedback with an eye to potentially adding some capability to the V3 provider that could enable this. e.g. Where(b => Doc.Field<int>("shadow") == 1)