Excel saying files have been changed by other users?

Stupid question below the users of this sub but I can't figure it out. We have a few users that use MSFT Office Home & Business 2021. If they have an Excel file open, put their computer to sleep, wake it and try to save the Excel file it gives them the message:

"The file 'file.xls' may have been change by another user since you last saved it. In that case, what do you want to do?

( ) Save a copy

( ) Overwrite changes"

Any ideas why it'd say the files have been changed by another user? The files aren't being accessed by anyone else. I've set the NIC's to not go to sleep. Can't think of anything else that might be causing it. Any ideas what else to try?

I'm wondering if this is just the behavior of this version of Office? Apparently their old version (I'd assume it was 2013 or 2016) didn't do this. They do have new PC's though so it could be something to do with that.