After invasions, all the drop ships used for the invasion are disappearing.

I'm at the prefect level, and for my invasions I intercept a legion transport and send it to the planet to be invaded, Planet A. Then I go to Planet A and request an invasion from Fleet Command. I get several transports this way. If I have three transports, I see 12 drop ships, so all is good. After the invasion of Planet A, I command each of the transports to go to the next planet I've lined up to invade next, Planet B. I go there and request another invasion and get another transport. I notice, however, that for Planet B there are only four drop ships available--the ones that come with the new (fourth) transport, which Fleet Command just sent.

It's as though we are leaving all the drop ships behind at the planet we just invaded. None of them were shot down by SAMs, and I waited until they all finished the transit from the surface to orbit, so I thought it must be a bug.