I have just integrated a Zigbee thermostat using Z2M. It shows up as a climate sensor and works fine, however it reports the temperature as an attribute which is not selectable in the area configuration.
I created a helper entity with the respective template and assigned it to the area, but it still does not show up in the area configuration as a temperature sensor.
Same issue I have noticed. I have helpers template and max/min and none of them show up as pickable in the area config. I suspect - it will only show temperature sensors attached to a device.
It shows up under the device you attached it to in the area configuration. It won’t show up as an entity attached to that area in that window. That’s how that window has always worked.