Does not necessarily have to be a user editing the same record, two people submitting unique records to same excel sheet open at same time will also lose all data being sent to excel. This happened to me years ago on an app that only submitted records to excel, had no editing or viewing of those records only new submissions. So, theres a chance it is something else sure, but just in case you thought it was safe as long as they do not access the same record simultaneously, unless it's changed in last couple years, even accessing separate records while another person or machine is accessing them will throw out the edits.
If you want to check another possibility, open your excel spreadsheet in o365 on a machine that is not the new one, see if maybe it says it is locked for use by someone, might get you moving in right direction
