Thanks, Basj. No other taskbar-specific apps. Only Start11. Only does it on this one laptop, a new Lenovo X1 Carbon. I compared the taskbar settings to another machine that doesn't exhibit these behaviors...
- Taskbar buttons should be combined on secondary monitors: (was Always), changed to Sometimes.
- Allow Taskbar to be resized: (was On), changed to Off
Those were the only two settings that didn't match the other machine. I'll give it some time to see if this resolves the problem.