For me it usually happens after waking up my monitors from sleep (on home machine this causes the primary monitor to connect back – when it goes to sleep, it disconnects from the PC, as if it was physically disconnected; on my work machine I'm not sure, it also seems to sometimes happen after waking up monitors from sleep, but none of those disconnect; installing the test version linked above immediately triggered the problem on both computers).
Both of my computers have 3 monitors, with centre being primary. Both are also set up with high contrast mode and use high DPI (one has all 3 monitors at 225%, the other one at 168%).
Sometimes the taskbar stops hiding on only one secondary monitor, but most often it stops hiding on both; also, sometimes when this happens, the taskbar becomes completely transparent, except for text and icons (it still receives mouse clicks in this state). The problem started with October update, and persists with KB5067036 (26200.7019).
Disabling and re-enabling auto-hide makes the problem go away for a while.