Async clipboard API doesn't work on Wayland

When e.g. going to https://github.com/qt/qtbase on Wayland and using the "Copy to clipboard" button in the "Clone or download" menu, nothing is copied and I get a Uncaught (in Promise) DOMException (can't seem to get more information).

I've tried to reproduce this using simplebrowser, but there, copying doesn't seem to work at all for some reason: I get a NotAllowedError: Write permission denied., but copying in response to a user gesture should always be allowed (and that's the case in my own project, without setting JavascriptCanAccessClipboard).