Grindelwald just couldn’t bring himself to kill Dumbledore. Those brief summer nights that he had convinced himself were nothing, just a way to a means, revealed themselves to mean so much more to him.
The only way for him to win would be to kill Dumbledore and that was suddenly not an option anymore, if it ever was one to begin with. In that moment, he let him guard down just enough to be disarmed.
Grindelwald never told his old friend about that moment of weakness, of what stayed him hand just enough to lose the loyalty of the Elder Wand.
Dumbledore already knew.