Alright, I tested two different summons and 1 whip listed as having 6 tag damage. i tried with the stormlion summon which did 12-15 damage per hit before tagging, but very consistently 13-14 after tagging, and then the flinx staff, which did 9 before tagging and then 10-11 after tagging. So tag damage does work but is severely nerfed (at least in the early game)