That's strange, but just unprotect it. As variant upload on Google Sheets and download back as Excel file, most probably protection is gone. Protection in Excel is only to protect from negligence, not to secure the information.
That sounds as if VBA is inserted in the Excel file. But to be sure you need detailed information. The Excel file (without sensitive data) would also be needed if there is a VBA code in the file, or to even be able to see which lock it is.
At the same time it can be what Mr. Baklan describes. So please provide detailed information in order to be able to submit a decent proposal for a solution.