This has happened before:
https://www.displayfusion.com/Discussions/View/multi-monitor-taskbar-scale-issue/?ID=48cf74c1-7adb-463f-96b6-78e4f3fd6c69

It seems to be happening with me. The text UI for the bottom right clock gets really big and overlaps itself, and the start menu stretches above the screen.
I've attached a screenshot.

Ah, I had StartAllBack installed, which caused the issue which I had completely forgotten about. My apologies!