v23 kept information about "installed games" in another location, in a different format.
v25 attempts to scan that, and update the information from the itch.io API. If the game can't be found, or the upload is gone - it won't show up in your library again.
I know that's unfortunate, and it's also why v25 stores install information directly in the installed game's folder. That way you can move around your games, reinstall your OS, change computers entirely, and import them afterwards with no problem.
edit: If you navigate to your install location from within the app, and use the "Scan locations for games", button on top you'll:
- See which games it finds
- If it doesn't find anything, you'll be able to see a journal of what it tried to import