I have a form for our program sessions that includes a subform for the purpose of listing participants.

The subform is a separate form, and includes three lookup fields:

For some reason, the parent form gives me an error if any two participants have any of those fields in common.  That is to say: if two participants are sponsored by the same society, or if they work in the same institution, the parent form throws an error when I attempt to submit it.

None of the fields are set to demand unique values. (Is that even a thing that can be done on a Lookup field?)  If I go to the subform directly and complete it outside of the parent form, it saves fine even if the sponsor or institution values match another record already saved.

I've sent a request to support for this, but I wondered if anyone else has run into it.  Is there an explanation (or better yet, a fix) that I'm not seeing?