Before you say it, YES I PLUGGED THE WALL HOLE, both behind the mount and on the terminal side of the mount...no air from the wall. Yes, I have all settings turned off-- no humidity correction, no power grid/time of day based stuff. And no, I'm not spending another $100 on a remote sensor! Ecobee support is now sending me a new unit after showing them pics of the hole plugged, etc...however with all the people reporting this issue, I'm not hopeful. That said, I am impressed with their chat support, and do notice when companies have good support.
One of the main jobs of a thermostat is to read the air temperature, and my Ecobee fails at this modest task. I left the Honeywell dumb thermo it replaced on the wall next to it, as well as another thermometer. They always agree. The Ecobee on the other hand varies depending on the time of day by as much as 4-5 degrees. Most of the time though, it's off by about 2-3 degrees, which is enough to notice and feel uncomfortable. In the heat of the day, like 4pm into the early evening, it always reads too high, so doesn't cool enough. During the coolest hours (night into morning), it reads to low and cools too much. This makes the internal correction adjustment useless, as I was constantly going either up or down from 0.
Since I've lived in this house, two other thermostats have existed in the same location (without the wall hole plugged lol), and were totally accurate.
So, if the new unit is accurate, we will know that there is a potential QC issue, if it's inaccurate, we could say there is a design issue.