It may be possible that it attempts to place the bottom block, which then creates the top block of the wooden door. The iron door, also being made of 2 blocks, only the bottom is deleted where the top remains until it is updated, which is the same time the wooden door attempts to place the top block. It is possible that it is a bug, but probably not. Try it with other multi-block items.
Yeah, pistons cannot push 13 blocks, so the same applies to the slime block (attempting to push the sand, with the sand pushing the other sand and so on). That's why screenshot 2 works.
It may be possible that it attempts to place the bottom block, which then creates the top block of the wooden door. The iron door, also being made of 2 blocks, only the bottom is deleted where the top remains until it is updated, which is the same time the wooden door attempts to place the top block. It is possible that it is a bug, but probably not. Try it with other multi-block items.