Scene is like this:
Condition Temp =< 21 Celcsius
Perform Heating ON
Effective period 05:00 - 23:00
If the temperature drops to 21 degrees WITHIN time, then "perform" is performed. However, if the temperature is already 21 degrees or below BEFORE the "effective period", "perform" is NOT executed with beginning of the "effective period". First, the temperature has to climb above the condition and drop again - then it works.
And by the way. It would be great if it was possible to specify a temperature range on / off in a scene (as with the Sonoff TH16). And as a trigger, the outside temperature related to the location is still missing. That would be perfect...