Hello,
I'm not sure why the view setting isn't being retained, but I will be bringing this to engineering's attention.
The setting is stored in the registry at:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Word\Last View
there is a sub-key called WordViewStateUsePageStackArrangement. If that key is set to 1, it is side-to-side. a value of 0 or no key at all, is Vertical
The key gets created when you change the page movement setting (Vertical vs. side-by-side). If you exit Word and delete this registry key, when you boot Word again, it will default to Vertical.
Let me know if this makes it work correctly, or if it gets "stuck" again. It sounds like something is going wrong with the writing or updating of this registry key.
Thanks,
-Mike