A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Microsoft only officially supports true AutoSave for files stored in OneDrive or SharePoint (the commercial version of OneDrive). Dropbox or “On My iPad” files aren't expected to have the same AutoSave functionality. But that does not explain the manual Save hanging.
There are multiple reports from this month with almost the exact same symptom. I believe this is an emerging bug issue on Microsoft side.
This is the example where a lot of others also mentioned the same issue.
I cannot find any public/official acknowledgement from Microsoft at the moment, but I fear this is something we will have to wait for them to acknowledge and fix. Please make sure you go to Help> Feedback to report this so that it gets to engineer/product team.