I faced the same issue when compiling the preview builds prior to their official release a few months back, and thought it was just a bug. I figured that maybe they had not yet fully restored all original features back into the engine at that time.
Then official 5.0 build came and someone told me that Screen Percentage may have been removed in favor of the new Temporal Super Resolution. I didn’t see where this made any sense and felt like its just another situation like what happened with Tessellation, where one thing cannot substitute another.
But anyway, you guys can do what I did to get around this and just download a free app that batch converts multiple images to a set resolution. The process is essentially the same as doing it with screen percentage except you are involved in the final step, which of course using an external application to do the downscaling.