I just filed a bug report. I also just ran updates on my Decklink card and NVIDIA gtx 680 (none available for Adobe). I can't re-create the tab away loss - but the open/close render loss does still happen.
Renders are no longer reliable in either Premiere or After Effects - I don't understand it. The renders were holding so well when you first released (even reverting to old renders on undo). I wish you would have just left it alone.