Office 365 vs 2016 should not matter.
If the specific key does not exist, try creating it.
Background: The normal Windows process is when you create a Windows User Profile a bunch of registry keys are setup under that ID. When Windows starts it copies your customized registry entries from your user profile to HKCU (H Key Current User). Then
you make customizations to HKCU during your session. When you shut down Windows normally, HKCU is copied back to your personal profile. If you do not shut down Windows normally, those customizations are lost. Sometimes Windows gets "stupid" and does not
grant itself the appropriate "Windows Access Rights" to add, change or delete some registry keys but it does not tell you this. Then you cannot make setting changes, but do not know it.
Here is a wild guess. Word cannot create the key in a way that is saved after you shut down Windows so Windows keeps resetting back to the wrong default. If you create the key, and set the value appropriately it may stay.
If creating the key in HKCU does not work, then you will also have to create it in your personal profile, which is copied to HKCU when you start windows.
You will have to look at the key after you set the desired option in the user interface to find out the correct value to save in it.