Thanks for your reply Katy,
We have had a similar issue with 3 users out of 400 as well, we also use OneDriveMapper and works very well indeed, all the Office apps and ie to office 365 also SSO, its very strange why 1607, LTSB 2015 and 2016 work floorlessly, nothing has changed with our configuration but still with version 1803 1809 single sign on we just cant get working properly, OneDriveMapper works just as well, but office apps do not sign in.
I'm guessing if the IE Zones were wrong the OneDriveMapper SSO would not work.
When you open Word or any office app, there is no SSO activity, click on file, open and no account shows, but as you can see the account is recognised and is shown in the top right of the app, when you click on Sign In it just goes back to this screen and thats as far as we can get.