Okay, I don't have a fill valve problem but since you've mentioned the Korky 528MP, I have a question.
The adjustable fill valve. Does it have a abnormally high fill tube rate? Is it possible that with the correct WL in the tank that wide open would underfill the bowl? If/does that happen where one has to increase the tank WL to allow the fill rate adjustment to have the needed range to properly fill the bowl?
I have a AS Cadet 3 1.6GPF and I measured exactly 1.6G as is to the marked tank WL.