Unprotecting sheet keeps reverting back to protected mode

I have a workbook that was created about a year ago which included buttons and macros to link to other pages (while the tabs were hidden) on a main "menu" sheet.

On the main menu sheet, there are two buttons to the right, the first labeled "Add action plan" which copies an existing hidden sheet titled "action plan". This in turn asks what to name the sheet, and adds hyperlinks in cell G18, G19, G20, etc. (as you keep adding new action plans) in the main menu sheet.

The second button is labeled "Delete Action plan" which deletes said copied sheets and the corresponding hyperlinks in G18, G19, G20 in the main sheet.

This main menu sheet is protected but when I unprotect it (with password), add action plans (3 times to test), then delete action plans (3 times), it reverts the sheet back to PROTECTED. Why is this happening?

Hope that makes sense.