The exact same thing is happening to me in CS:GO. I've always had preserve aspect ratio on. When I opened it the other day it was on full panel somehow, but when I changed it to preserve aspect ratio it changed to black bars as normal. Now it's happened again but no matter what settings I change it stays full panel. I just finished clean wiping all AMD software and reinstalling it, and the problem persists.
This is an issue with the drivers after 17.4.2 (from what I've gathered), and it's still an issue with 17.5.2 (Radeon-Software-Crimson-ReLive-Edition-17.5.2-Release-Notes, known issues), which is the newest driver at the moment.
Thankfully, AMD has acknowleged the issue, and they have stated that they are working on fixing it.
Please hurry, AMD, this is frustrating.
Here is the most updated thread on this issue I could find.
Hmm, I guess it's time to downgrade.