If Tiamat dies after the two minute timer to kill all heads expires, from DoTs, the instance finishes, and you get the rewards. However, when you leave the instance, you have to abandon the instance and take the 30 minute penalty.
Basically, the death of Tiamat outside of the 2minute head stage does not trigger the end of the instance.
LEVIATHAN--19.3k Metallic Dragonborn Guardian Fighter Swordmaster Loadouts
Guild--And the Imaginary Friends