Better Thermostat entity unavailable

Hello, I’ve set up better thermostat for three of my radiator valves, combined with external room temperature sensors. That works great, except for every now and then, when one of them becomes unavailable.

So the problem is that the better thermostat entity is unavailable and my automations with it fail. The actual Aqara Zigbee TRV is still available, but obviously doesn’t change its set temp anymore.

To solve this, I have to remove the whole BT device and recreate it. I have already exchanged the TRV, but that doesn’t help.

How can I find out what’s causing this? Can it be caused by the zigbee device loosing connection temporarily? Not that I expect that to be very likely, the TRV and the sensor are only 4m and a thin cardboard door away from a powered router device.

And a second related question, because I always find out about these events much later. I’d like to know immediately. I have a daily automation that notifies me of disconnected devices. It’s a template as follows:

service: notify.mobile_app_cph2449
  message: >-
    There are {{ states.sensor  | selectattr("state", "in", ["unavailable",
    "unknown", "none"])  | map(attribute="entity_id")  | list  | count }} problems with the following devices {{ 
        | selectattr("state", "in", ["unavailable", "unknown", "none"]) 
        | map(attribute="attributes.friendly_name") 
        | list 

But it never picks up on the unavailable better thermostat entity. How should I change it to get notified of that? Or a second template specifically for this maybe.