Noticed that mobs don't drop XP when killed by TNT. The minecraft-wiki list that TNT should drop XP.
For confirmation yes Im only using one TNT, no 2nd tnt to destroy the orbs that drop after the first TNT blows. Im getting normal drops like string, bones, gunpowder, rotten flesh, etc.
Works fine for me. Make sure you are igniting the TNT with flint & steel, not with redstone.