Hi Sam,
Based on your description, you have issue that can’t add/hide/unhide worksheets. And the issue only happens to this specific workbook.
Refer to
Hide or show worksheets or workbooks, we would like to confirm that does this workbook contain VBA code? If worksheets are hidden by Visual Basic for Applications (VBA) code that assigns the property xlSheetVeryHidden, you cannot use the Unhide command
to display those hidden sheets. If you are using a workbook that contains VBA macros and you encounter problems when working with hidden worksheets, contact the owner of the workbook for more information.
Please also check whether you have protected this workbook via
Review>Protect>Protect Workbook. As you cannot hide or unhide sheets in a protected workbook.
Thanks,
Rena