Can you replicate the issue on apex.oracle.com?
When you do it, please share the credentials in order to find out the problem.
OK, I found the issue. As usaual it was between the chair and keyboard (so ashamed). Seems like I can't read as the message was not about primary key but about filling a FK. The IG region was in fact detail and FK field was not populated automatically. I have put there default based on the item and now it's fine.