There's a weird inconsistency in vertical droppers that face each other when powered in a very specific way.

You can recreate it by building the attached circuit. Put a single item in the bottom dropper. When you press the left button the item moves up into the top dropper and then back down to the bottom one as expected. However pressing the button on the right (after pressing the left button) will not activate the bottom dropper as it should on the first try. A further press will succeed. Looks like a bug to me.