Proton Mail and Pass both show a blank screen when switching between the app and another recent app while multitasking, so Proton Wallet should too. This seems to be a fairly common feature among most financial and crypto apps and it's something that Proton Wallet should have to avoid exposing account balances when the app is in the background. Yes, you can hide the balance by clicking the eye icon, but given that every banking app I use now does this I think it should be a standard feature here too.