Excel prompts that file has been changed by another user

I’m in the habit of pressing Ctrl-S to save Excel files but Excel 365 nearly always throws up this prompt. Nobody else is working on the file although it is in a group folder. I don’t get the same with Word. Any ideas? It’s rather annoying…

I select “Overwrite changes”. I can then press Ctrl-S to my hearts content - the prompt doesn’t occur unless I come back to the document (say) 5 minutes later and save it again.

Windows 11 Pro and pretty recent (like month) version of Nextcloud with latest client.