In Blender 2.81 and 2.81a no matter what I do and in which mode I can’t undone my actions. Control+Z shortcut does nothing, undo history is empty.
Perhaps the Undo Steps value has been modified?
Edit > Preferences > System > Memory & Limits > Undo Steps: 32 (default)
I adjusted my Undo Steps to 0 (zero) and reopened my current working file. Performed some actions then checked Edit > Undo History and observed the same behavior you’ve shown.
Returning the value to 64 (what I use) and reopening current file resulted in the Undo History working again.
I have value setted up to 32. Still same issue.
Is this possible that my laptop specs are causing this? On 2.80 I couldn’t move bones because Blender had bug when working on Intel HD graphics 4000.
So I’m using:
Intel Celeron N2840
4GB RAM
Intel HD Graphics 4000
SSD Drive