Who? Bisping was a +600 underdog, Nate was like +375, Usman stepped in on the same card with the same notice and had a close fight with a heavy favorite, Masvidal fought Usman without a camp and went to decision and took very little damage, then got his jaw punched into the 3rd row with full camp, GSP took his rubber match with Hughes on short notice and won easily.
There are plenty of examples of guys doing as well or better as a late replacement, many who have both full camp and short notice fights with the same person to compare, but Volk Islam is the only one I can think of where they have both and the underdog did worse in the short notice one.