Hi Nate,
I've had that happen to me a couple times. I figured that is what the gladiator going inactive between the minute or two I'm putting the team together and when I click to begin. It's relatively rare, but maybe if there was a confirmation question of "do you really want to start without a full lineup" or something like that to let you know that a teammate has been dropped off the selection list. Not a huge deal, just more of a minor/infrequent annoyance.