HP71B RES bug?

[Albert Chan]

I had assumed RES = result of last expression (or last DISP expression)

However, RES may be affected by user defined function.

>10 DEF FNK(K,X) = K*X
>20 DEF FNR(X) = FNROOT(1,2,FVAR*FVAR-X)
>RUN

>1 + 0
  1
>FNK(2,RES) + FNK(3,RES) + FNK(4,RES)         ; expected 2*1 + 3*1 + 4*1 = 9
  32                                                                 ; got 2*1 + 3*2*1 + 4*3*2*1 = 32
>FNR(RES*2) + FNR(RES/2)                             ; expected √64+ √16 = 12
  10                                                                 ; got √64 + √4 = 10

Is this a bug ?