When clicking on a combat mission to enter, there can be a lag in the button press. If you press it twice before it loads the character selection screen, you will enter the battle with whatever random 5 toons defaulted on the selection screen.
This has happened to several folks in different guilds using a variety of devices.
Bottom Line: Be careful and slow down when selecting combat missions. Click once and wait for it to load.
Simple fix is to do what was done with Territory Wars and make the character selection screen blank in TB to prevent you from entering with the wrong toons for those too quick on the trigger or those experiencing lag between screens/button presses.
The better fix is to correct the lag issues AND also implement the same change from TW to make the character selection screen blank.
Nothing like clicking on a battle in Geo TB and seeing your imperial troopers load into battle instead of your Sith Empire.
This is even worse in later phases when every point counts and doubly important on Ship battles in P4. If you get lag that causes you to enter with Tarkin Capital and random ships, it's game over immediately.
Fun Note: Still managed 3/4 with Imp Troopers but it could have been way worse and has been worse for several other folks.