Please note that we've updated the Mount & Blade II: Bannerlord save file system which requires you to take certain steps in order for your save files to be compatible with e1.7.1 and any later updates. You can find the instructions here.
It is funny the surrender "mechanic" even exists when there is absolutely no reason the player would ever choose it...
I would suggest that, when choosing surrender, you are able to, through dialog, negotiate troop release, so that a good chunk of your force either gets transported to your garrison or donated to a nearby friendly vassal.
Come to think if it, I think I'll mod this in since my crappy computer can't play the game but I can't seem to step away from this game