An update on this topic which had been going on (in several threads) for 5-6 years…
Give Ceasar what belongs to Caesar: I have downloaded PL10 trial version, and look and behold… The issue is FIXED!
The viewer now (at least with DeepPrime render activated) displays colour and sharpening correctly at every zoom level above 25%!
It’s better than PL9, and HUGELY better than PL8.
There still is a very minor, almost undetectable, shift at 25% and below, but it’s really a non-issue now.
All this while performance/lag has not got worse on my machine. Actually, it’s gotten slightly better!
It took years, but now I am satisfied, and I will definitely upgrade this year (last year I skipped exactly because they still had failed to fix the issue).