I installed 6 x ecobee light thermostats and all of them are missing auto and heat devices:
supportedThermostatModes : [off, cool]
I've tried manually adding heat by switching to a virtual thermostat and go back to Ecobee, but it did not work and I got something like your thermostat does not have a heat function on debug logs. Same as when I send a heat comment.
Thermostat heats normally, connections are fine, and it is not API as I've additionally tried Ecobee Suite, and all worked fine except CPU load, and that one creates so many tasks and everything slows down and fails with 6 thermostats. So i had to go back to the built-in Ecobee integration, which at least works fine.
I have ecobee v3 light too, all 6 of them. Auth was fluent and everything works well except missing state modes.
I have many of these errors on the logs of the Ecobee Integration app, and I am not sure if they are related:
java.lang.NullPointerException: Cannot invoke method multiply() on null object on line 1052 (method setHold)
One other difference may be that I am not using a furnace, so I am using a cold and heat pump for HVAC air conditioning. So the 0B wire is connected instead of W. I have no idea if Hubitat cares, just a side note, not if it makes a difference. The ecobee hardware itself can switch between heating, auto, and operating modes. It is only Ecobee integration that does not have these modes.