I have searched for an answer and have not found one so maybe it's a bug?
This happens ANY time there are 2 types of rockets on pylons. That includes single missions, quick missions, instant action, select in mission planner, add while on the ground.
If I have one type of rockets on 3 and 5 and another on 2 and 6, I am only able to access one of them. If 3/5 has 10S and 2/6 has something different, I will get "# 10S" or whatever it normally says on the corresponding pylon. I would also get 10S at top. Let's say the other pylon, 2/6, has a type that comes with 7. For the corresponding spots on the diagram it would say "7---". I also would have no way of clicking on them at the top row (osb 7). Similar scenarios have been the same way but on opposite pylons. Maybe I am doing something dumb.
Also, for one of the single missions or instant actions, it allows for rocket practice so it gives me 19 on each wing. When I go to the mission editor for other missions I am not able to pick any rocket type that allows for 19 rounds.
I will take a screenshot later this evening so hopefully that will help explain it.