For the fat path, it seems every 4th of 5 levels the cow is actually losing max hp instead of gaining it. i saw it happen twice. not sure if it always happens, but when i am doing my farm runs it becomes obvious.
Edit: I just saw the same lower Max HP when share leveled from 31 to 32
Edit: ok i think it might have something to do with loading a save. I just loaded a save, and my shark had more hp. then when i leveled up it went down some, but was higher than before i loaded a save.