I have been using a MS Teams site since March - we have one excel document which is regularly updated/amended by 2 people. Up until this week all changes have been saving 100% ok. As of 2 days ago when we make changes to this document and close it down it initially saves the changes. If a colleague makes changes, exits the site and I then review the document those changes are present - great its working we think.
1 hour later one of us goes back into the document and voila its reverted to a former version minus the changes made earlier that day!
@Debbie_HarrisTU I would suggest doing a save as for creating a copy of the same files and testing both the files... maybe something is broken in that old excel file that creating a new copy will fix that.