Wow, that is an impressive amount of work you did, if it makes you feel any better – and it probably doesn’t – I’d have felt like screaming, too.
A bug of this magnitude is just unacceptable, in my opinion. If you can’t trust the software, what good is it?
I ran into a similar situation. I like to shoot close-ups of flowers, and so I’d use an Auto Mask on the flower, duplicate that mask, and then Invert the duplicate so I could update the background separately (typically making it blurrier and darker to make the flower stand out a bit more).
Almost EVERY SINGLE TIME I did this, I’d adjust the background, and see the flower get updated instead. Huh?!
I’ve racked my brains, but I can’t think of one single good reason why this should work this way. I just think it’s a bug. And it’s a huge one for me. My suspicion is that when the second mask is inverted, not everything is getting switched to the background. Man, that seems like a really easy/obvious/simple bug to find and fix, if so.
Brian