To be more specific with the way of issue reproduction:

WTR:

1. Launch the game;

2. Make sure the CB is killable with 1 key;

3. Start the CB fight few minutes prior to the CB reset (got the recent reproduction starting 8 minutes prior to reset);

4. Make sure not to finish the CB fight before the CB reset time comes;

5. Deal enough damage to finish CB;

6. Note that the CB didn't die (cause it's unkillable);

7. Finish the fight after the reset time.

Actual result: damage dealt in the fight is counted for the reset CB, clan members are getting x1 chest.

Expected result: if a fight that started prior to the CB reset and finished after the reset time results into dealing more damage than the CB HP level at the beginning of the fight, that damage shall count to the previous day CB, clan members shall receive x2 chests.

Note: player's damage counts to the reset CB if less damage than CB HP level was dealt as well. It's contraversial if that's an issue or not:

Current behavior leads to a chance of player not getting a better chest (though x1 anyway) but saves the key if the player didn't actually needed extra damage on CB, wanted to just finish it.