Sometime around sept 20 2021, all my ecobee devices in two locations went offline. One house has 2 thermostats and 4 sensors the other 1 stat and 2 sensors. I gave it a day or so as others on the ST FB group were reporting a similar issue. Some resolved it ok by re-authenticating. I tried on one house and it gave me an error message. The other house I tried adding a new ecobee device - it prompted me for login, and then re-authenticated and it added those devices back. No go on the other.
Today I resolved to remedy the situation. Tried again with house 2 no go. I went into the IDE for house 2 and behold there were TWO entries for each ecobee device - an older one that was offline, and a new one that said PLACEHOLDER that was online. I tested deleting one of these and then replacing any instances where it was used in an automation with the new PLACEHOLDER. I also use ACTION TILES, WebCoRe, and simple device viewer. I ultimately swapped out all the old instances on house 2 with the new placeholder all seems to be fine now. However, house one still uses the older instance and no placeholder instances were added there. Weird. Anyone posting here in case someone searchs and comes across a similar issue. ST is doing SOMETHING on the backend again that caused this apparently.