I just tried to use bunq.app for sending a transfer (mostly because copy-pasting the IBAN was easier that way). I was asked to enter my PIN, but instead of filling the PIN-dots typing numbers started searching the page, a feature which is called “Search for text when you start typing“ in the Firefox settings. Needless to say, it shouldn’t be necessary to change browser settings to use the web app.
I guess the reason for this is, that the web app does not use a proper password from but instead some weird javascript magic. I don’t think that’s good solution, I wouldn’t be surprised if this messes up accessibility as well.
I would prefer to confirm web transactions using the mobile app anyway (at least if it also was used to login an therefor is available).