In Remnant Cognizance 29, the mission can fail to update if you leave your fleet in a different system. about endless-sky HOT 3 OPEN

Although not for this specific mission, I can confirm this behavior.
As EvilBarney describes, if you leave escorts in another system and those escorts happen to carrying any of the "reserved" cargo or passengers, the mission will not proceed. One problem is we can not assign cargo and/or passengers to specific ships. It's completely random.
The other problem stems from "go get something" missions are adding the cargo and/or passengers when you accept the mission. What ever you are being sent to pick-up should not be "added" until after you reach the destination planet.
I don't think we have that ability at the moment.
I discovered this in a different scenario. I accepted a mission to go get something and thought "while I'm on my way I'll take some trade goods to sell". I assumed I was traveling empty but found I didn't have any free cargo space. It was already taken up by the items I was supposed to go get, but I hadn't even left yet.

from endless-sky.

Did you check the ship info panel and see the mission cargo there before or after taking off?

from endless-sky.

Did you check the ship info panel and see the mission cargo there before or after taking off?

Both cases looked the same - the flagship Bactrian was the only ship in the fleet with space for the 280t Reserved cargo, which showed up in its info panel before and after taking off. (The rest were Shield Beetles with half that capacity each.)

While landed, the crew panel showed 20 passengers in the fleet. I had only the essential 70-ish crew on the Bactrian with all other bunks free, but I don't know how passenger berthing is handled in practice - any of the Shield Beetles would have had the space as well.

from endless-sky.