Thanks @Shegs @rxunique - your comments seem to confirm our hypothesis that our Upsert doesn't play well with "generated always". It seems we always send the null value regardless of whether it's in the changeset. When you try to insert, tries to insert the null as the filter and that's where it throws an error. And it seems like MSSQL 'IDENTITY' has the same behavior.
I've surfaced this internally for our eng team and will keep this thread updated with any new info.