Thanks for the additional info. I was able to reproduce this by de-authorizing SmartThings on the Honeywell TCC portal. There is a similar open bug report and I’ll add this additional information.
If you go to https://mytotalconnectcomfort.com/portal/MyAccount, do you see SmartThings listed under “Subscribed Applications”?
In terms of resolving the issue, if you are only using the Classic app, I believe if you delete the thermostat devices you should be able to remove the Honeywell SmartApp. Alternatively, in my experience I was able to remove the SmartApp from the new app under “Linked services”. If those aren’t options or don’t work, ST support can remove the connection.
Once removed, there shouldn’t be any issues re-adding the integration.