WoWInterface - Hook fires for every element except the first one

Hello. Currently i am working on redesigning the challengemode frame. I noticed something i cant quite get the grip on.

On the challengemodekeystoneframe, you can add a keystone which will trigger this function:

Lua Code:

function ChallengesKeystoneFrameMixin:SetUp(affixInfo)
    if (type(affixInfo) == "table") then
        local info = affixInfo;
 
        SetPortraitToTexture(self.Portrait, CHALLENGE_MODE_EXTRA_AFFIX_INFO[info.key].texture);
        
        if (info.pct > 999) then
            self.Percent:SetFontObject("SystemFont_Shadow_Med1_Outline");
        else
            self.Percent:SetFontObject("SystemFont_Shadow_Large_Outline");
        end
                
        self.Percent:SetText(("+%d%%"):format(info.pct));
        self.Percent:Show();
 
        self.info = info;
    else
        local affixID = affixInfo;
 
        local _, _, filedataid = C_ChallengeMode.GetAffixInfo(affixID);
 
        SetPortraitToTexture(self.Portrait, filedataid);
 
        self.Percent:Hide();
 
        self.affixID = affixID;
    end
 
    self:Show();
end

Now since i want to change the look of the icons, i tried to hook into that function like this:

Lua Code:

hooksecurefunc(ChallengesKeystoneFrameAffixMixin,"SetUp",function(self,affixInfo)
        print("shot fired")
        self.Border:Hide()
        self.Portrait:SetTexCoord(0.15, 0.85, 0.15, 0.85)
        --m_fontify(self.Percent,"white",miirgui.medium)    
    end)

THis worked out just fine, except for the very first element it should work for. It just seems to always ignore the first element, no matter how many elements there are :( I meanwhile found a workaround by hooking ChallengesKeystoneFrameMixin:OnKeystoneSlotted() by shouldnt hooking the setup function work too?

Blizzard ChallengesUI