This issue was reported on Aug 05, 2023. It is fixed on Aug 11, 2023

The reason it happened:

The Power score is partly calculated based on the levels of heroes and units, provided their durability is greater than 0 (zero). As a result, the score will decrease if a hero or unit's durability becomes zero after a battle. Currently, we only update the score after claiming a hero or unit in the current version. This may create an illusion for players that they have lost score after claiming a unit (though the score calculation is working as designed). 

The fix:

We changed the system to update the score after each battle. If the durability of any unit reaches zero, its Power score will decrease accordingly. This separates the score calculation from claiming a new unit.