I made a screen recording doing this in the help vault. You see that the behavior is erratic. It sometimes works, suddenly it does not. So the screen recording shows several events where I successfully can copy with the context menu, but suddenly, it stops working and then paste does not work as well. You can find the recording here, as I did not see a way to upload it here directly.
Also experiencing this behaviour. This also applies, when using keyboard (CMD+C/CMD+V).
In edit-mode, tried to mark text using mouse and keyboard. CMD+C does not copy text to clipboard (verified by pbcopy | cat) nor is text pasted to editor with CMD+V