You might be on to something - SBD is 1.8.9 at its base, 1.9.0+ features are disabled - throwing splash water bottles at fire to put it out, dispensers equipping pumpkins on player's heads, etc. If you're trying to place a trapdoor above a block (not connected to the side of a block), this might be the reason. *can't find a source that confirmss or denies this, however.
It might also be a farmland bug? Farmland's upper hitbox is smaller than other blocks, that might be why you can't place a trapdoor on the upper half of a farmland block.
On what block are you failing to place trapdoor? In what orientation - side (upper/lower half), upside down, top, freefloating...?