I figured. It's calling
LFGDungeonReadyPopup_Update() in its
OnShow handler, which hides it immediately if there is no "proposal" data. You can disable it by running the following line before
StaticPopupSpecial_Show(). You will need to reload your UI before trying to queue into anything in order to restore functionality.
Code:
/run LFGDungeonReadyPopup:SetScript("OnShow",nil)