If you equip the Isadoras “SET BELT” you get “2 SET BONUS” ALLREADY.
Currently not having the helmet to test which is bugged or what causes the bug.
Right now i can gain the “2 Set” bonus with a “Single Set Item”.
And a “3 Set Bonus” with “two of them”.
I can’t see your screenshot by clicking on that link, could you please check that the link is correct/it’s configured to be public?
I’m not able to reproduce this-- you’ll see the 2 and 3 piece bonuses in grey text even if you just have the 1 item. The bonuses will be green if you have them.