So I’ve dug in some more and things are a bit strange.
EF9 and CosmosDB throw the same error as our provider. In fact there is an open ticket for it on EF at Support `HasConversion` to primitive collection types · Issue #33688 · dotnet/efcore · GitHub
I thought originally it would work because my test on EF8 Comos worked just enough to load and save data without throwing but I suspect that was more luck than intent.
Collections in EF are a bit more involved that regular types given that EF needs to snapshot and compare them for the whole change-tracking etc. That happens up at CLR level not at the DB conversion level so it knows nothing about the types you’re trying to map from up there. Not to mention that even if it did support it you’d never be able to do things like .Where(d => d.Additions.Count > 0) as Count can never be translated as it’s a property on an “unknown” CLR type.
I don’t have any insight into the EF teams priorities but I suspect this is quite a large request with at-best a rough experience. It might be easier for them (or perhaps us) to just natively support HashSet.