Crosses between mallard derived and muscovies usually take a few days longer than mallards, but not as long as muscovies. I had a hen once hatch 2 crosses and 2 pure muscovies. Luckily she stayed in the nest until the muscovies hatched.
They may have slowed their movement while the heat was a bit lower, but one hatching is a good sign that the lower temps were not enough to effect the hatch.