Thanks for the info Andi.
You were on the latest version of FP4 FPOS but a new one has just come out, FP4.SP2G.B.079.20230624. It may not be available yet on your phone, but keep an eye out for it. Go to
Settings > System > System update
As I understand, you are using the Proton email app, but to access the mail of a Gmail account. Is that correct? No other account, just the Gmail (Google) one?
I rather insist on getting this info correct, as it’s a rather unusual setup. Normally people either use the Proton app to work with their @proton.me account, or they would use the standard Gmail app to work with their @gmail.com account. Or else, like Amoun, they use a completely independent app like Fairemail (nothing to do with Fairphone by the way), often to work with several different mailbox providers.
It’s worth knowing, that the Proton app is designed in particular to work with a @proton.me account, in order to manage special encryption features. Its use with a non- proton.me account may explain the momentary disappearance of the content when you open a message.