That is nice, that is also why it is called a bug. Bugs are, by nature, unpredictable. The fact is that it happens on both dedicated and APUs, it has existed since before APUs were even a thing. Fuck, it did it with my 3850 when it was brand new... The bug has existed for years, it has been documented so many times it isn't fun. However, it is now worse on APUs. That doesn't mean APUs are the cause of the problem.
Yes, troubleshooting requires deductive reasoning. So lets use some of that. Again, using your mastery of deductive reasoning, please tell me how something that works in every driver before 14.12 and stopped working with 14.12 and later, is a problem with the chip and not the driver? Please explain how changing JUST THE DRIVER causes the issue to happen or not, but yet it isn't a driver problem? Walk me through how your deductive reasoning brought you to that conclusion.
Holy crap that worked! The driver now doesn't underscan automatically when lowering the resolution! The slider is still locked out on my APUs, but it doesn't matter because I don't need it since underscan isn't automatically enabled anymore!
My deductive reasoning says that if a registry setting fixes it, it is probably a driver issue, but I'm sure someone will use their mastery of deductive reasoning to say it is an issue with the APUs and GPUs, AMD's drivers are definitely not to blame here...
P.S. Note that the fix is from 2013! So we know the bug has existed in AMD's drivers at least that long...a simple registry entry the AMD is failing to write with their drivers and they haven't been able to fix it in at least 2 years(I know it to be a lot longer)... Gotta love those AMD drivers.