To reproduce:

Existing Configuration:

• Launch a profile previously created

1. Cannot launch: The game directory is invalid or inaccessible. Existing: directory

Create New or Edit Configuration with the Game Directory Option:

• Create a new Configuration, or edit an existing Configuration, with a custom Game Directory (I've tested this path: C:/ .)

1. Go back to "edit" the new/edited Configuration: Game Directory is switched on, but the text box is blank.
2. Try to launch with the new/edited Configuration: The game directory is invalid or inaccessible.
3. Check %appdata%/.minecraft/launcher_profiles.json: "gameDir": is not recorded for the new/edited Configuration but those previously created with custom Game Directories.

Edit: I previously thought the bug only exists on previously created configs with custom Game Directories. Turns out, affects those using default directory too.