The dragon you transfromed to is Krystenax. There is a known bug associated with him.
Did you also get an extra turn from Krystenax’s cast? Did the cascade that follow also result in another 3-gem match? That seems to be to way how this bug usually happened, after casting Krystenax, killing all enemies, and summon Silver Drakon.
In the meantime before the fix, try not to cast Krystenax as a finishing move.