DJNZ instruction running simulator of DScope doesn't work properly

Observing action single stepping through below code segment reveals that DJNZ instruction in
div_loop doesn't decrement R6. On first loop execution it adds 16. The next loop through decrements.
Susequent loopings causes it to have an add other times a decrement. Running version 2.30 of uVision.

Code

div_OK: MOV R1,xh
MOV R0,xl
MOV R2,#00
MOV R3,#00
mov xh,#00
mov xl,#00
MOV R6,#16

div_loop: clr c
mov a,R0
rlc a
mov R0,a
mov a,R1
rlc a
mov a,xl
rlc a
mov xl,a
mov a,xh
rlc a
mov xh,a
lcall sub16
mov c,ov
cpl c
jnc div_1
mov xh,zh
mov xl,zl
div_1: mov a,r2
rlc a
mov r2,a
mov a,r3
rlc a
mov r3,a
djnz R6,div_loop
mov z3,r3
mov z2,r2
mov z1,xh
mov z0,xl
mov r4,xh
mov r5,xl
clr OV
ret


sub16: mov a,xl
clr c
subb a,yl
mov zl,a
mov a,xh
subb a,yh
mov zh,a
mov ov,c
ret