[SDPhantom]

I'd suggest hooking or rewriting StaticPopup_SetUpAnchor().

The popup frames act as a frame pool, you'll either have them show at random locations or have your anchor overwritten if you statically set one of them.

For example, this uses a secure hook to change the position of the root frame only. Others will spawn under it as is default behavior.

hooksecurefunc("StaticPopup_SetUpAnchor",function(frame,i)
    if not StaticPopup_DisplayedFrames[i-1] then
        frame:ClearAllPoints();
        frame:SetPoint("TOP",UIParent,"TOP",0,-135);
    end
end);