I have only heard of a dominant gene for muff/beards, not a recessive one
So I would be looking really hard for a possible muffed parent, such as a different hen that laid confusing eggs, or a different rooster that had access to the hen at some point. These things happen more often than most of us would like, and hens can store sperm from a previous mating (sometimes for much longer than anyone would reasonably expect!)
If your chick does have a recessive gene for muffs, it is new (not well known/studied), but is also present in BOTH parents (because that is the only way the chick would show the effects of a recessive gene.) Of course new mutations are possible, but they are not very common, so they are not the first explanation I would suggest.
If the mother is a Rhode Island Red mix, do you know what else is in the mix?
Does either parent have a beard? Some Silkies have beards, but muffs & beards are actually caused by the same gene.
Is there any chance that one parent did have a muff, and the feathers just got picked off by other chickens? I have read of that happening sometimes, and it would be one way for a chicken to seem to have no muffs but still have the gene and be able to give that to its own chicks.