Yeah the same thing is happening in my game with my fiancé. I'm hosting and have unlocked it but for her, the bottom row of mutations only show a total of two mutation slots instead of the normal four slots that should be there. The only way she can get it to show up is if we start a brand new game and after spending 115+ game days in the game and building a brand new base with mushroom bricks, we really don't want to have to start over again.