There could be one thing that is the reason - is there room inside the entity your in to actually drop the item? Its recently been changed again that items on you don't count for the entities weight and volume limits, but there is a check if/when you try to dro it.
Maybe go outside or to another ship or something and try again.
If theres no room on the entity in question for a certain item then the check removes the 'Drop' button from that item. The item still appears on items there is room for, I just checked, so as Elias suggests that is most likely the issue and there is no bug.