I have a similar problem to yours I'm trying to render games in a lower resolution on my 4k screen to make them playable. I also used cru to make a custom resolution of 3200 * 1800 but It never streches to full screen. I always end up with a native 3200 * 1800 on my 4k screen so with black bars on all 4 sides. I used this without problems on my nvidea card but its not working here. I will try the older driver you said and report back.
As I understand, solutions to this problem will never happen, only one way out: I switch to Nvidia, there are no problems, everything works with any resolution and aspect ratio of custom resolution.