Watchtower isn't showing available passkeys on one device

Ok...I'm going to rewind a little on the comment "signing in with passkeys works perfectly" in my previous post, above. The passkey I generated this evening worked perfectly at the setup stage but when I went to use it a little later, the prompt wasn't showing up fully (see image). The prompt did fully load (visually) once, but it wasn't functional and the sign in button could not be clicked. However, after restarting my browser (Safari), it works fine on the site and it seems consistent🤞🏼. Might be a bug that needs looking at?

...and I've got away from the main topic, but at least it's not on someone else's post 😅. Soz