I'm not going to read through all the replies, but I will say that I posted a thread about this exact situation a few weeks ago. The defense that was credited with a fumble recovery was Jacksonville, if memory serves.
Posters chimed in with opinions, and some research was done.
Basically, according to NFL rules & scoring:
1. A muff is the touching (but not possession) of a then "live" ball.
2. The player who touched it is not charged with a fumble (since there was no possession).
3. If the opposing team recovers the ball, they
are credited with a "fumble" recovery (even though technically there was no "fumble").
This happened several times this year, and I'm sure in years past (since this is not a "new" rule). I think Houston had a pair in the same game, though I'm sure nobody noticed since their defense probably wasn't on anyone's fantasy squad.
Since the NFL scores it as a fumble recovery, I think you're obligated to give the points to the defense until you change your rules to specifically prevent it. After looking into the situation, I decided to leave my scoring "as is" rather than have to manually track every fumble all year long. It doesn't happen enough to warrant that type of effort.
Oh, and edit to add: