Can't apply "Page Title" style

[Bl0tto]

Apologies if this is answered, if it is hopefully the question title helps others find it.

I can not find out why the "Page Title" is not changing, is something corrupt in my OneNote installation? I can see this error in the log, only when 'Page Title' style is applie:

Running command ApplyStyleCommand ..styles not applied

I checked the reg-helper, everything is "OK".

I have had the occasional issue of OneNote having some problems 'cleaning up' but I have done a repair of both OneNote (offline) and OneMore with no issues. Might quickly try an online to see if anything is different.

Any idea what I can do/ look for? I can try Uninstall/Re-install?

NOTE: I have has GEM and Notetastic installed previously, but have been uninstalled, and deleted any files I found. Just in case there may be a clash there.