I'm pretty tired of the buginess with SmartThings. Most recently they made an update that broke the link between SmartThings and Google Home. In the past I solved this by creating new email accounts and re-linking.
How does one totally remove old email accounts? Support is never available, and once deleting an account from inside Samsung's web interface you can't recreate an account with the same email.
This sounds like one for our Accounts Team. You can contact them directly on 0800 995 1079, or alternatively on your phone go to Settings > Accounts and backup > Accounts > Samsung account > 3 vertical dots (top right) > Help > Contact us.
Hope this helps!