Hello Carlo..
I just got the SD40 delivered during the week, Ive noticed the same problem with the autofill and filltoarranger function going haywire.. Ive sent an email addressing this issue with AJ, so hopefully he could share some light.
The SD40 is an amazing little box, easy to navigate and the styles and voices are incredibly real.. the Twist style especially, i just cant help but keep The song going
Ps In the mean time just turn it off, press the fill in button followed by whatever arranger variation you like, just a small extra step to take until the problem gets sorted hopefully..
Ky