After a bit of investigation, the way to fix this would be to modify MergeMaskIntoAlpha to pass in the actual resolution scale and the requested size, and only mask the overlapping pixels. Currently MergeMaskIntoAlpha only compares overall total pixels to check for masking. Not clear on why it works proportionally to the size requested currently.
Download licensee repro from here - [Link Removed]
Open in editor and enter PIE
A screenshot will be taken but an error message pops up:
[2021.09.06-17.05.25:985]LogOutputDevice: Error: Ensure condition failed: bMaskMatches [Link Removed] [Line: 137]
If you then go to BP_Highres and change the resolution request to 3840*2160, or 1920*1080, the screenshot works as expected (although there still appears to be masking of the background, but this may be desired behaviour?)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-125930 in the post.