A family of Microsoft word processing software products for creating web, email, and print documents.
Please disregard the Windows-based response from PanyongT. If you've attempted to do what was suggested you've already found that none of those steps can't be followed on a Mac.
The key question is whether there actually has been a change that would cause the prompt. Even if that's the case the prompt shouldn't appear repeatedly but this is something that does happen from time to time for some users. Not having any idea what may be included in "all the usual troubleshooting tricks" there may be some that you haven't tried.
Add-ins are one possibility: There are some that may affect the template, especially if they are not fully updated. Have you determined that no add-ins are being used? If there are, either update them of remove them to see whether that resolves the issue.
One procedure that often remedies this is to open the Normal.dotm template in Word using the File> Open method, type a character or two, backspace to delete them... Do Not use Undo, then Save & Close the template. Restart Word afterwards, then check to see whether it continues. If not, it may be that there is a problem with the template, itself. The next step would be to have the program replace it. Let us know before you attempt to do so.
There is a Preference in Word> Preferences - Save to prompt before saving Normal template which is in effect by default. In most cases it should be left in effect in order to prevent unwanted changes from being saved, but it can be turned off as a temporary fix.