[Lombra]

You should set the main border on the original frame instead creating your own. You don't have to, but in this case the new border frame does not have its points set, so it won't be visible. Since it already has a backdrop, you'll also want to retain the background bit. Try this:

local tooltips = { GameTooltip, ItemRefTooltip, ShoppingTooltip1, ShoppingTooltip2, ShoppingTooltip3, WorldMapTooltip, }
for idx, tooltip in ipairs(tooltips) do
	tooltip:SetBackdrop({
		bgFile = [[Interface\Tooltips\UI-Tooltip-Background]],
		edgeFile = [[Interface\Buttons\WHITE8x8]],
		edgeSize = 3,
	})
	tooltip:SetBackdropColor(TOOLTIP_DEFAULT_BACKGROUND_COLOR.r, TOOLTIP_DEFAULT_BACKGROUND_COLOR.g, TOOLTIP_DEFAULT_BACKGROUND_COLOR.b);
	tooltip:SetBackdropBorderColor(0, 0, 0)

	local iborder = CreateFrame("Frame", nil, tooltip)
	iborder:SetPoint("TOPRIGHT", -1, -1)
	iborder:SetPoint("BOTTOMLEFT", 1, 1)
	iborder:SetBackdrop({
		edgeFile = [[Interface\Buttons\WHITE8x8]],
		edgeSize = 1,
	})
	iborder:SetBackdropBorderColor(0.5, 0.5, 0.5)

	tooltip:SetScale(cfg.scale)
	tooltip:HookScript("OnShow", TooltipOnShow)
end