Venom draught missable?

I finished the campaign a second time on cruel and am no longer able to find Servi to claim the venom draught reward for the 2nd playthrough, where the choices were Grants 25% increased Stun Threshold, Grants 30% increased Elemental Ailment Threshold or Grants +5 to all Attributes. When I go back to Ziggurat encampment, the town is no longer there. Is this permanently missable?