It doesn't seem to happen with a test account, though it's hard to say for sure since it's intermittent. I have tried disabling various things without any success yet. Sort of hoping this symptom would ring a bell with someone.
I'm not using BetterTouchTool but I do have a variety of things installed: Adobe Creative Suite seems to install a bunch of resident stuff, Little Snitch, iStat Menus, Magnet, Smoove, Yoink, Pastebot, Timing. I've tried eliminating the ones that are easy to quit (ie, not Little Snitch), but I'll keep at it. I found some remnants of Karabiner Elements installed and uninstalled that but no luck.
Thanks.