I have a similar/same issue where, perhaps when docking/undocking may laptop or plugging in another monitor, the fences "docked" at the bottom that typically expand upward (where the header sits at the bottom and moves upwards to expand the fence) seem to forget they are "docked" at the bottom, and the header stays mid-screen and the fence then expand downwards when I hover.
I have to move the header right to left and let go, and then the fence re-"docks" to the bottom of the screen and the header sits at the bottom of the screen again as expected when the fence is collapsed.
This is extremely annoying. Please fix, devs!!