1. Download and extract the "1315831 repro" project into a Perforce Workspace
2. In P4V Add and Submit the Assets, Project Settings and Packages folders.
3. Open the project and set up the Perforce version control in the Project Settings (while using the same Workspace as in step 1)
4. In P4V switch the Workspace into a different one
5. Get the latest version of the project
6. Find the "SampleScene.unity" and "Cube.prefab" files, select them and in the right-click menu select "Change Filetype"
7. Add the Exclusive Checkout option (+l) and press OK
8. Submit both files
9. While still using a different workspace, checkout those 2 files
10. Go back to the Editor and open the "SampleScene" Scene from Assets > Scenes
11. Make a change to "Cube.prefab"
12. Save the Prafab

Expected behaviour: the Prefab is still modified on disk to prevent lost work
Actualy behaviour: the Prefab is never saved.

Reproducible with: 2018.4.32f1, 2019.4.22f1, 2020.2.5f1, 2021.1.0b9, 2021.2.0a7

Notes:
-The behavior is different for the "SampleScene.unity" file, after saving the Scene, a popup appears telling you the Scene can't be checked out but the Scene is saved and if you look in Explorer the file is no longer Read Only.