Attempting to make a flush piston door with a keycard system seems to break, but I made one earlier and that one does work. Not sure if it's a flaw in my redstone, or if it's a bug, but I'm thinking it's a bug based on how I built my previous door and how I opened it with a redstone torch a few minutes before I found the glitch and after finding it.

Here's what breaks:

The door stays open, but the top middle pistons extend and stay extended, though breaking them releases the side pistons. The bottom middle pistons don't extend but get stuck, and breaking the block they seem to be sticking to releases them to be pushed by the side pistons.

See this YouTube video for more info (my video exceeds the 10mb file size limit): https://youtu.be/Pod61zI9VCQ

Update: It seems to be an issue with block updates, as placing a redstone torch behind the pistons makes them work.