The bug

If using a dispenser with a flint and steel in it with a TNT in front of the dispenser, the TNT will be ignited, but the flint and steel will not be damaged.

Note that this is the flint and steel activating the TNT, and not the redstone signal that activates the dispenser; this happens when you have a button that indirectly powers the dispenser.

Cause

else if (world.getBlockState(blockpos).getBlock() == Blocks.TNT)
{
    Blocks.TNT.onBlockDestroyedByPlayer(world, blockpos, Blocks.TNT.getDefaultState().withProperty(BlockTNT.EXPLODE, Boolean.valueOf(true)));
    world.setBlockToAir(blockpos);
}

does not attempt to damage the item.