I've adjusted the touch sensitivity and it gives no improvement in Calibre. And as mentioned, no other webpages have this issue.
I managed to luck into the right combination of keywords to find others with a possibly related issue:
https://www.mobileread.com/forums/sh....php?p=4153779 (related bug report:
https://bugs.launchpad.net/calibre/+bug/1943517)
Like them, I'm running a Samsung device, and like them, I was able to get it working on Firefox (though, also like them, I greatly prefer Chrome on Android and it is a more convenient experience for Calibre use). However, whereas they appear to find their highlights working but causing errors, any of my highlights that
do work are perfectly fine, and sync just fine. It's just that I usually can't make a selection in the first place.