Is there a known differenz between richt-click / copy and Ctrl + c?
Sometimes either one copies selected text sometimes it doesn´t. Is this a x11-wayland bridge problem?
It will depend on the app and how it handles those two events, the menu copy and the key ctrl-c. Usually an app would not break the assumption that these are the sane.
But a developer is free to break that assumption.
Which app is not following the copy/ctrl-c convention?
I think that more than one. Try to do in Konsole Ctrl + C and after do a Ctrl + V in Kate.
After try in Konsole Right Click + Copy and in Kate do Right Click + Paste.
Regards
Konsole is a bad example.
It assumes that in a shell the user wants ctrl-c to be the sigint key.
Now as it happens I reconfigure konsole to use ctrl-c/ctrl-v for copy and paste. On my systems it works 
i use ctrl-y for int and ctrl-b for quote.
I copied from a VSCode extention (SQLite) to a Claude prompt in the same VSCode. at least I thought I did. only the keyboard shortcut copied anything. KDE Connect get in the way? I dunno . it is a bit annoying and not without danger as on paste it pasted the old content unreplaced content from the cliboard.