Trapdoors When the Block Below Them Updates

At first I thought that this was just a redstone thing. I was on a zombie apocalypse map, using open trap doors to trick zombies into falling into a pit where I could grind them. Then I added some pistons below, but whenever I activated the pistons and they pushed up, the trapdoors closed, and they didn't reopen when I turned it of, so I had to open them manually. Then I tried to place and break blocks under them, and that also closed them. To reproduce the issue, dig a 2-block-deep pit, and put an open trapdoor under it. It will close. Then find various other ways to update the block underneath the trapdoor while it's open. It will close every time.