This occurs when you have more than 8 soldiers available to deploy for an 8 person mission.
Steps to occur:
Fly two ships with a total of at least 9 soldiers to the same mission point.
Enter the Deployment Screen, where deployed soldiers are selected
Deselect one of the initially selected soldiers and select a different soldier for the mission
Click on one of the soldiers to enter that soldier’s loadout screen
Hit the back button or “Esc” key to return to the deployment screen
At this point the characters selected to be deployed has “reset” to the original 8.
This was bugging me because I wanted to check each of my soldiers loadouts and try to put together a good loadout for the mission (Pandoran Lair) that I was about to start, but each time I got back to the deployment screen my choices had been reset and I had to reselect them.
I’m guessing this is not the intended behavior, so I think it makes sense to report it as a bug. Thanks!