Tnt + trapdoor won't break blocks

When TNT is lit in the block above a closed trapdoor, the TNT falls onto the trapdoor, at which point it is in the block that the trapdoor is in. When the TNT finally blows up, it damages nearby mobs and players but does not break blocks, similar to when it is detonated in water. This does not always work, when the TNT from the screenshot detonated it did in fact break blocks.