What version of Word are you using? (from the About Word window)
Current version is 14.6.7
If you still have the same after updating, it might even be OTHER software that you have installed.
This being Microsoft (
), have you tried uninstalling/reinstalling Office 2011 from your original installer?
And, then be sure to try the Help menu/Check for Updates to make sure that your Office is fully up-to-date.
And, now I see that I have wasted your time. The split view function, provided by OS X, is not supported by Office 2011 apps. (I was thinking the "split" function within Office, which is not the same thing. My mistake
)
It's Microsoft's problem. Maybe it is related to the fact that Office 2011 apps are still 32-bit (?)
Nothing you can do, except to use Office 2016 apps (or ignore split view mode when using Office 2011 apps)