The problem is solved. I don't exactly know why it's fixed. but the fields in the headers are now updating.

Running the code you posted, the pane on the left (search results) and the pane at the top (ribbon and tabs) turned white, and the screen flickered quite obnoxiously for several seconds. About six of the properly constructed field insertions turned into errors. (I inserted all fields via the GUI, if they are improperly constructed it's Word). I'm sorry, but that's what your code did.

(Word 2016/Office 365/Windows 10 Pro x64)