I have been using Google home with smartthings for months with mostly no issues. Since last week I have been unable to add any new devices though.
Previous I would just select smartthings in the home app and the it would let me log in and reselect which devices to add without deleting what I already had. Now the only option I get is to unlink/remove smarthings.
This has been posted in the above mentioned thread.
This issue was introduced when we launched the new Google Assistant integration a few weeks back and only affected users who unlinked the old integration and linked to the new integration, or installed Google Assistant for the very first time.
However, it took a while for us to figure out the cause since it related to only custom device type handlers. We do thorough QA on our WWST devices but there is no way for us to test all custom smartapps and device handlers. Since it relates to custom code, it only affects a smaller number of power users and not the majority. As a reference, we got about 10 support tickets on this which makes it hard to track down.
As mentioned before, the official fix will deploy today or tomorrow (it has passed our internal QA) Lars Engineer at SmartThings
I’m honestly not sure that thread describes the same issue, though they do seem related. I checked and I still have Google Assistant in my smart apps and the only option in Google Assistant settings is still to unlink.