Save file is attached, along with a video showing the behaviour.

In the video, the first item drop is a single presses of 'z'. The others are holding 'z' while dragging the cursor across cells. You can see it is somewhat inconsistent: moving fast enough drops one item only. Changing the hotkey for ...

When dropping items into the rocket silo's 'rocket inventory' slots using the 'drop item' key (default: z), the first press of 'z' puts two items into the inventory. Subsequent presses of 'z' add a single item. Expected behaviour is to always drop one item only.

Reproduce:
Build a rocket silo.
Open ...