I recently purchased a TH6320ZW2007. I had the wiring rerun with a 6/18 cable from the HVAC area to the thermostat. I look at the ZWave status of the mains, I’m getting a unknown reading.
I checked the voltage between the ‘G’ and ‘C’ wire and I got a pretty steady 27.5VAC reading. Seemed correct to me but I’m not really deep in thermostat electronics.
I also believe my T6 is running off battery and not the ‘C’ wire. I’m seeing lots of losing connection and then regain them. From the forum search, that’s often a sign of running off of batteries when this device shouldn’t be running off batteries.
I’ve been using the T5 forever, and it’s been rock solid, but it’s Wi-Fi without batteries.
I’m not familiar with the T6, but I would try removing the batteries and running it without them. I’m fairly certain you don’t need batteries for it to operate. While they can act as a backup, I think Honeywell added batteries for older HVAC systems that might not have a C wire.
Most thermostats accepts 18 to 30 VAC on the C wire. Your previously measured 27 V falls comfortably inside this range and is fine.
Lastly, enable RSSI if it is not already enabled and see what the signal level is like. You could also try rebuilding the Z Wave routes. I have had to do that sometimes when I move my Z Wave plugs around.
I suggest the latter because, to me, this sounds more like a signal level issue.