Please see the below 2 screen shots; when a colonists is given a command to give a partial stack to a pack animal, a blank screen appears instead of the normal slider bar which allows the quantity to be selected. I believe this also happens when a colonist is given the command to "pick up (x) of (item)".
In the below example, I'm giving Martin a command to give some Herbal Medicine to the alpaca next to the medicine.
In the below example, I'm giving Martin a command to give some Herbal Medicine to the alpaca next to the medicine.