Reproduction:
Use /setblock x y z wooden_door 0 replace, where x y z is the location of the bottom half of an iron door.

Expected behavior:
The iron door should disappear, and be replaced with a wooden door.

Current behavior:
The iron door disappears, as expected, but the wooden door is dropped as an item, instead of being placed.

Other notes:
Replacing the top portion of a complete door of either type with any block, including air, causes the door to drop as an item.
Replacing the bottom portion of a complete door with any block, including air, causes the door to disappear.