Yep that schema all looks fine, so this might be an actual bug! Recently we've had trouble getting support to elevate these type of issues. Maybe a moderator or your hubspot rep can elevate this for you?
So we've seen a lot of strange "happenings" with custom objects in the last ~3 months. I'd assue this is because HS is pushing new/modified functionality.
Could you post the objects schema? With that we might be able to get some insights.