Have you tried setting the region in Max itself, that should carry across to slaves (at very least as a workaround, til we hear whether it's expected for VFB regions to carry across - btw afaik every machine will start with some general "everything" and then just focus on the region, looks like the slaves may have sent data during this initial phase, and they are respecting the region, because there is only parts of the image filled in from the slaves not the whole image)