Pistons create half doors

I created a slow comparator clock as shown on the image and connected it to a sticky piston. And then I thought: "Can pistons actually move doors?" - (would be very interesting to make something with that) and placed an iron door onto the piston. It broke and I thought: "Well, okay... " - but placed another one and, as it seems, I did it at the right moment and the bottom half of the door broke, but the top one stayed (see the second picture).

This is in normal vanilla world with cheats on and without any resource or data packs.