Why is my Mother-32 distorting when patching into the VCA CV input?

Because the CV input does not have the physical limitations of the volume pot, patching more than +5v into the VCA IN allows you to drive the output VCA into clipping. The VCA CV has one other interesting difference from many other patch points in the patchbay in that it is normalled to the VCA MODE switch, so it responds to incoming voltages differently depending on that switch’s position: 0 to +8V in EG mode and +/-5V in ON mode, summed with signals from ON/EG switch.

If you want to avoid this clipping, please attenuate the voltage you're patching into the VCA CV input. You can use the Mother-32's own VC MIX utility as an attenuator if you want!