Not sure, even in the Light Novels it is not explained clearly how their stats work on a level up. I suspect their base stats are there and present, just not visible on the level 2 sheet. I can't see them entering the dungeon with 0 stats, that's like asking to be killed and they do not start over at floor 1. I suspect their base level stats are there and maybe build on to their current level kind of like a passive skill or in this case passive hidden stats.