In R2020a on windows 10, if you adjusted the system scaling factor, e.g. to 125%, then every time you open Colormap Editor, it will be bigger than last time, as noted here. This bug can not be reproduced on R2018a, at least on my PC. So this bug may be newly introduced in R2020a only.
Anyway, I contacted Matlab Support and they told me it is "fixed" in R2020b. I installed R2020b. It turns out Colormap Editor is totally re-designed. So it is "avoided" or "left-over" rather than "fixed". The bug mentioned above disappears in R2020b, however, many more "bugs" (or unwanted features) appears. Here are steps how to reproduce one of them.
Only after I close all the figures and reopen one, I can edit the colormap limit. Also, the newly-designed Colormap Editor is much slower than the older one when opening. This is really frustrating.
I have attached a short video to demonstrate how I reproduced the bug.