Prior to 1.13 (I've not played the 1.13 snapshots), a dispenser a fire charge facing a block would apply its' effect to the air block past it. A dispenser facing a block with an air space after it, above a block of netherrack, would light the netherrack.  It now appears to light the block to the left or right of the direction the dispenser is facing when ejecting into a block.