I've noticed some wonky things as well after trying to restore saves using the backup feature. Best way to restore "backup saves" w/o actually using the backup feature:
1. Save your current game
2. Go to your saved_games folder and click on the saved game you want to "backup"
3. You will see a list of folders w/ a bunch of DAT files below them
4. Click on the auto-save folder at the very top
5. Copy all of the DAT files in the auto-save folder and paste them into a separate backup folder
6. Whenever you want to restore your save, simply copy and paste those DAT files back into your league folder and overwrite the DAT files already there
Here's a video explaining the process from OOTP themselves. This works when trying to restore your saved games if your files ever get corrupted for whatever reason and you can't choose "Continue Game", but it also serves as a way to manually backup your leagues w/o the wonkiness of the backup feature...
OOTP 22 Tutorial Series - Autosave Restore Tutorial