I have an excel file which I been using since last year. I have a few sheets that I keep hidden so other users don't have access to them. To add more protection I protect the workbook structure, and I have not had any issues until now. Every time I protect it, it only last like one hour before it unprotects itself. I been protecting it saving and closing it out, and later on when I reopen it the workbook is unprotected. I notice the issue started when I started using PowerPivot, but I'm not sure if that is related. I need help figuring out how to keep the workbook protected, as I have many users using it and I want to prevent them from changing anything or opening any sheet that is not needed by them.
If I may recommend you, attach a file (without sensitive data) or a photo, inform about the Excel version, operating system and storage medium. This way you can get a solution proposal much faster.
From afar, I cannot understand how the workbook/sheets were locked with the previous information. Think that here it is appropriate to send the workbook as long as it does not contain any sensitive data.