When you say it "goes into update mode," could you describe in more detail what you are seeing? What I see is that the TOC becomes shaded. That is the behavior on my settings when I am in a field, and the TOC is a field.

However, it is not updating.

If it is updating on your computer, try starting Word in safe mode and see if you get the same behavior.
Open Office apps in safe mode on a Windows PC - Microsoft Office


I assume you added a Back button to the QAT because that is not part of the default.