For some reason I can't type 'F' when using Stadia via Chrome, I've tried incognito mode, in case any extensions were playing tricks on me and even tried different games (my main issue is when playing Elder Scrolls Online, but I also tested the input in Rock of Ages, in case it was a game-dependent issue).
I can type 'f' and any other letter I can think of, but specifically not a capital 'F'.
I also have this problem. I noticed when I was trying to enter shift codes for Borderlands 3. I can't even type F with caps lock. only lower-case F. I'm not sure if this is causing some codes to work or if some codes are just not valid on Stadia (but are valid on Steam, I tested one... but I don't own the game on Steam).
Try another browser, ideally without ANY plugins installed... Brave browser works with Stadia
You can configure plugins to work in incognito mode as well, so that doesn't guarantee that the plug is not to blame...