Just reporting back to say that yes indeed Proton Pass can fill app credentials in too. I have no idea why it didn’t work when I tried it last month. I was trying it out as my Bitwarden subscription was due for renewal and I was seeing if it was worth migrating to Proton Pass. Sadly my renewal has come and gone now so will now be something I need to revisit next year.

Anyway, thank you for confirming that I was indeed incorrect.