As with so many things in aviation, "cut and dried" cannot be applied to this situation.
I have flow through volcanic ash that apparently fell "out of a clear blue sky".
After taking careful avoiding action well to the windward of a visibly erupting volcano, I was ambushed on final by ash falling out of a blue sky over 100 miles upwind of the volcano. It looked like the haze/Sahara dust which we are accustomed to. It did not last long and we left after a short delay. I understand later on it came back and coated the island with about 1/10 inch of fine white pumice type dust.
So "see and avoid" may not always deal with the problem. It is more complicated than that.