locked GrapheneOS warned me that a banking app tried to access the telegram messaging app clipboard. Could it be??
I think the warning says "APP pasted content from clipboard", where APP is the name of the app that did read the clipboard content. The warning doesn't say which app the clipboard content was originally copied from, so I am a little bit confused over you saying it said it came from Telegram. It shouldn't have said that. It doesn't for me.
I think it would be interesting to see the exact wording of the warning you saw, if you can reproduce it.
With that said, yes, any foreground app can read the clipboard content, and GrapheneOS warns when this happens. Unless I misremember, the GrapheneOS developers have expressed interest in implementing a way to block specific apps from reading the clipboard, to cope with situations exactly like this one you are experiencing.
Running the app in a secondary user profile is of course also one way to prevent this, since each secondary user profile has its own clipboard, there are no clipboard sharing between secondary user profiles. It is just owner profile and hidden space, and potentially work profiles that have clipboard sharing.