Slithering Dead / Venom Draught BUG

lvl96 witchhunter here backtracking some missing progress.
figured the slithering dead quest is completed for both normal and cruel.

normal shows +25% increased mana regeneration bonus on the venom draught map icon.

cruel shows nothing. slithering dead quest is completed....

checked Servi and tried to redo the quest... no luck.

any help will be greatly appreciated!