That is not a muff. It is forced touching. Here's the rule, (Rule 6-2-4, if you're scoring at home.)
ART. 4 . . . Any K player may catch or recover a scrimmage kick while it is beyond the neutral zone or the expanded neutral zone, provided such kick has been touched by a receiver who was clearly beyond the neutral zone at the time of touching. Such touching is ignored if it is caused by K pushing or blocking R into contact with the ball or it is caused by K legally batting or muffing the ball into R. Such catch or recovery by K beyond the neutral zone causes the ball to become dead.
It is my understanding (at least in the NFL) in the case of batting the ball if R picks up the ball of their own accord (the batting didn't cause R to touch) then even if R fumbles K can not recover. I'm pretty sure I saw this happen on a replay and it was one of those "really heads up plays" that was praised. Is this the case in HS?
First, the NFL rules are very complicated and I have no idea what their rules are on this, but what you describe is sort of backward from high school rules.
First, in addition to the Rules, there is a list if FUNDAMENTALS of FOOTBALL. These are the foundations upon which rules are built.
Fundamentals II-3: If R is first to touch a scrimmage kick beyond the neutral zone, a new series will be awarded the team in possession at end of the down, unless there was a foul before the kick and the penalty was accepted, or there was a double foul or there was an inadvertent whistle during the kick following the touching by R.
You already know this -- this kick crosses the line of scrimmage and it's touched by R, so there is a mad scramble. Barring anthing mentioned in the caveat at the end of the citation above, whichever team gets it, keeps it.
So let's say K is first to touch a kick beyond the line of scrimmage. (you ignore a leaping lineman trying to block such a kick, but that's not what we're discussing here.) If K touches it but does not possess it, a bean bag is dropped to mark that spot. Technically, there is a different spot anywhere K touches it beyond the line, for all touches that occur before R touches it.
Once there is a spot of first touching, R always has the option of taking the ball at that spot, first and 10, no matter what else happens (provided they don't foul). So an alert returner could see the ball hit a K player, see a bean bag come out, and realize he's basically got a free play. He can pick it up and run just to see what happens. If he fumbles, even if K recovers, R can take the ball at the spot of first touching.He backtrcks and loses 30 yards, no problem, take the ball at the spot of first touching.
That's probably what you were thinking of with the NFL rule.