Hello Kevin,
sorry for the late response. I've fixed the problem. Here's what it was: all those forms with the problem had they Primary BizObj's Requery type set to type 2. As soon as set it to 0("Don't Auto Requery"), the problem's gone. I don't know how I've missed that when I was comparing working form with the broken one but that's all it was.
Thank you for your time and help.
Regards.
Igor