The only real solution is to not change the resolution. Period. That's what causing the problem.
BTW. After generating Optimized Media is finish there should be summary window was it successful, or not. Or if there were some files skipped.
EDIT: Officially it's getting to be very annoying. I've one SSD 500GB drive that is dedicated for video editor caching. There is nothing else. The project that I am working on at the moment requires more than 500GB for Optimized Media when it's in original, 4K res.
Why... WHY optimized media require that much storage space? I get the point, that this codec is "light" for a CPU/GPU, but until this BUG is still around it's a pain...
I am on 16.1_b2