From "Photographic Theory and Practice-Part 4:Monochrome Processing" LP Clerc,DA Spencer,RE Jacobson 1971 p496:
"Metol......is soluble in water 4.5 percent at 15 C........... (from a concentrated solution) the free base is precipitated when sodium sulphite is added."

So,IMO,it could be that more concentrated sulfite solution forms the free base on the surface of any added metol particles that stops them from dissolving.

IDK if that is the correct explanation, never come across this before.