@Felthry I've seen a message that related to this that pops up on at least a few browsers I use, and as far as I can surmise, it's something weird to do with an OS' handling of its own clipboard, vs Google app's handling. Weird permission sets regarding where data is stored, even temporarily.
@emanate Hmm, so the online app can't access the clipboard? That's odd, but okay... I could swear we've seen online things that you could click a button to copy things from before.
@Felthry Yeah. My guess is, using the (former) menu used Google's clipboard internal to the app, while the keyboard shortcuts use the OS, since those are the standard keys for that. Possibly they're getting rid of the Google clipboard entirely.
For reference, the messages I've seen have been using Google Sheets within an old version of Internet Explorer, on a Windows 7 machine.