Steps to reproduce:
1. Create a new Unity project
2. Open Edit->Undo History
3. Generate some Undo events
4. Select the 2nd item from the top in the Undo History window
5. Click on "3 dots" button in Undo History window and select Show Latest Action Last

Expected result: Only one separator line shown after switching to Show Latest Action Last
Actual result: Separator line between the first and second item in the undo list is not removed, there are two separator lines now in the list

Reproduced with: 2022.1.0a8, 2022.1.0a4 (Undo History was added in 2022.1.0a4)