When I try to change the formatting for numbered sections, it reverts automatically.

I am writing my chapters in separate documents. I would like to have the second chapter start all numbering with 2.X but it automatically is set at 1.X.

Microsoft Word for Mac
Version: 16.84 (24041420)

Screen capture: https://drive.google.com/file/d/1Jyx8su3VfTd5Tc9IxDSe_eyQpDKlvJ03/view?usp=sharing

I looked online and found instructions on how to change the starting point. Format -> Bullets and Numbering -> Outlined Numbering -> Customize -> Level -> 2 -> OK

It works, but when I move away from it, even just to scroll, it automatically reverts to 1.X.