The setup is a dropper with one or more item(s) with a button on the back. Above the block is some opaque block, and above that is a powered piston (facing somewhere other than down) with the power source off to the side. Activating the dropper with the button will work once, but (after restocking the dropper if necessary) hitting the button again does nothing. Presumably, this is because the dropper is quasi-connected to the piston, and therefore is continuously powered regardless of the button (although perhaps it works the first time because the button sends and update). However, even after removing the piston and nearby redstone, the dropper will still not work (suggesting it is still somehow powered by the now-removed piston). Replacing the piston or the dropper seem to sometimes, but not always, reset the issue such that it works for a single button press but not on any subsequent ones. I have reproduced this multiple times, albeit in a single world and only in the latest release. It does not seem to effect dispensers.