[DahkCeles]

I would try inserting the following at line 12:

local function initCommon(frame) -- old line 11
    if (BackdropTemplateMixin) then
        Mixin(frame, BackdropTemplateMixin)
    end
    frame:SetBackdrop(backdrop) -- old line 12

After quickly scanning the code, my assumption is that every frame with a backdrop has this initCommon function called once only before any backdrop functions are used.