Ghasts, Blazes and Wither Skeletons all take fire / lava damage since the 0.15.90.7 update. The changelog stated "Nether mobs no longer lose fire resistance" which I think was a fix for MCPE-12467, but has left the mobs stated in the title with no resistance to fire and lava.

When flying too low towards the lava lakes or very close towards lava falls, the Ghasts make hurt sounds and turn orange/red as if on fire and eventually die.

^{Title and description updated by Mega_Spud}