I am playing a co-op game with another player. I am hosting the game and I was able to unlock mithridatism after killing my first 5 wolf spiders. My other player has now killed 20 spiders. We have tried killing them over multiple days, without dying, and all in the same day without dying.

Same. When I‘m host, it unlocks after 5 Wolf Spiders for me. When I join my bf‘s game at the other hand, I can kill as many as I want and nothing happens. Same happens vice versa to my bf. 

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.

Now after further investigation I think it is an issue with the two new mutations. We just beat the brood-mother and I unlocked mom genes and she did not. Also, in her crafting menus the recipes for the club and mask don’t show either. I hope they fix it in the next patch.

same problem here, can't unlock mithridatism as a client on xbox series

Im on pc, playing as Hoops. My mate plays as Willow

My mate and i take turns hosting.

I've downed 18 wolf spiders now according to peeping.  and level 2 of Mithridatism still has not triggered. 

Quote ""I've downed 18 wolf spiders now according to peeping.  and level 2 of Mithridatism still has not triggered."" ～ThumpieBunnyEve
Updates:  I took a copy of the shared cloud save. played it solo, killed 5 wolf spiders, the Mithridatism lv2 finally unlocked this way. Now sharing that game instead.

Solved～sorta.

-Hello

I apologize for the frustration regarding this mutation unlock while playing Grounded. I recommend submitting a support ticket, so we can investigate this issue. 

Obsidian Support

Please include the following information:

1. Platform (Xbox Console, PC Game Pass, PC Steam).
2. Multiplayer or Single Player Game (if multiplayer, can you tell us how many players in total in the game, and their platforms if known)
3. Difficulty level
4. Description of issue.
5. Last thing you did in the game before the issue occurred, if applicable.

Thanks, and hopefully we'll be able to get this resolved!