The Keyword List doesn't update properly when you undo a move of a keyword. To reproduce:
1. Make a new catalog.
2. Create the parent and child keywords A > B.
3. In the Keyword List, drag B to the top level.
4. Do Edit > Undo Move Keyword.
5. Observe that B still incorrectly appears at the top level.
6. Restart LR.
7. Observer that B now correctly appears under A.
Tested on LR 11.2 / Mac OS 11.6.2.