I have a Hansgrohe as you describe, with the Talis trim, and really, one thing has nothing to do with the other.
On mine you only push in the front ring to override the high-temp limit, but other than that is turns freely from one stop to another.
You set the temperature with the front ring, and then the other ring with the handle sets the volume, or amount, of water you want.
The thermostatic cartridge does what it is suppose to do: Keep the temperature set where you want it to be. Therefore if the water heater starts sending cooler water, the cartridge automatically increases the hot, and decreases the cold.
The best way to make sure you don't run out is to make certain you're using a water saver shower head, don't set the temp control to the hottest setting and don't set the volume to maximum.