Page 46 of the manual explains what's happening.
"PolyBrute can generate a random patch for you by choosing sounds A and B from different patches, changing the morph knob value, moving the result to sound A, and then repeating most of that process a random number of times. The generated patch can then be tweaked saved as a Snapshot, or saved as a new preset."
So it's always going to fall at a place between two pre existing patches. It's not randomly applying 0 to 100% settings to all parameters and so can avoid nonsensical sounds.