I've been in several Lockdown games now where, for whatever reason, someone has left the game before the match finishes (usually near the start). Asking players to finish out the match when their team is clearly at a significant disadvantage is not fair or fun.

It's possible this already happens and I can't tell because all the game math is SECRET, but there should be some sort of mechanic that gives the smaller team some compensation, such as increased health or reduced respawn times. You run the servers, you can easily collect statistics and tweak the amount of compensation until having a player leave doesn't significantly affect your chances of winning (compare to games where no one leaves but which had similar disparities in score at the time of the departure).

Once you get it balanced correctly, you could even start games with an odd number of players if necessary to prevent someone being left in the waiting queue. So it will be a useful feature even if no one leaves.

Alternately, if it's early enough, maybe let another player into the match in progress to fill the slot (with a discounted entry fee or something).