No, by rule the "impetus" is the original fumble by R and the muff by K does not change that. Review Rule 2, Section 13 - Force, Article 2 and Article 4.
The original fumble was not going to go into the endzone as the ball was "almost at rest at K/4" when A's muff subsequently forced it across the goal line.
2-13-2 says "unless a new force is applied to either a kick, fumble or backward pass that has been grounded."
Article 4 (2-13-4) discusses when ball becomes dead with no player in possession. That seems to answer the original question here, but when we expanded the discussion to if the ball went thru the endzone and OOB, I don't understand why A's force wouldn't be to blame.