Hi Owen,
I won't guarantee that upgrading your macOS to Big Sur will fix this issue. To be honest this sounds like a long shot to me as well.
There were 2 M1 chip users reporting the similar issue in this thread and if I remember correctly, the M1 chip mac come with the Big Sur OS.
So if they are experiencing the same issue in Big Sur, then in my idea, the system OS is not the root cause for the issue here.
During this period, I've managed to setup a macOS Catalina 10.15.7 system. However I still can not reproduce this issue.
When zooming in the Word application, it won't jump to the random page.
If you want to see how it works on my side, please let me know and I will send you a video.
I will try involving some related resources and see if they know anything related with this issue.
Meantime, could you try setup a new local user account on your Mac and see how it goes?
Set up users, guests, and groups on Mac
Also, I am not sure if George asked this before, have you installed any add-ins in your Word application?
Regards,
Alex Chen