To Reproduce:

1. Put attached project inside a workspace & set up a connection to Perforce.
2. Submit everything in the default changelist.
3. Click Connect.
4. Quickly open scene "main".
5. Unity will be in a state where it doesn't automatically checkout a scene if saved and other assets such as materials can be edited and saved without first needing checkout.
6. If you leave the project open, after a few minutes Unity will return to a working state where assets can be checked out properly.