Apparently, the Fitness Trainer job can force remove your traits, and it doesn't even ask the player.
So after a session the Fit trait get "upgraded" to the Athletic Build trait, but I had both to begin with. So, instead of nothing happening. It eats the Fit trait and I lose it and gain nothing for it. I'm a little frustrated at the loss, considering I just got it, and it's gone already.
I'm going to stop playing for now considering the state the game seems to be in.