I can reproduce this and it does not happen if you immediately kill start 11 and return to windows taskbar it's immediately fixed. (or if you restart explorer from start11 menu)
It's very intermittent, but happens more often when switching between multi-monitor setups (eg when moving from desk a to desk b to desk c with a laptop) without rebooting or forcing explorer restart.
Stopping S11 will restart explorer though so would instantly fix it.
I am thinking this might be a bug in Windows rather than us and would expect it to show up the same without Start11 but as you say it is intermittent which makes it harder to test this.