I thought the game auto-saved entering or exiting the crypt, but just loaded an old save to try it, and it doesn't.
The cutscene conversation starts less than a second after you exit the quest reward window (at least in the North American Xbox version), but there is a brief period where you can use the dragon stone. Apparently reloading the area causes the ex-slayers to revert to their behaviour and dialog from before the crypt was opened. Re-entering the crypt, etc, doesn't help, so I don't know if there is any way to re-trigger the cutscene.
I think you will either have to just skip this, or email support@larian.com, and see if they can fix your save (though they will not be back from vacation until next week, so their response time might not be that great).