I'm sure whether this will help anyone else, but it does seem to be behaving consistently for me right now, through multiple logout, sleep, and reboot cycles. Then, I made the following changes on the Position tab, to pull the taskbar into the right place:
Yesterday, I pulled in the 21H1 feature update (taking me to build 19043.1237). I have a single monitor running at 2560x1440 resolution.
I've been using TaskbarX v1.6.3 for about a year with no problems on Windows 10 Pro.