Ah, just noticed - S. Metabisulfite is recommended for the clearing bath if you are using Permanganate/Sulfite bleach. S. Sulfite is recommended for the clearing batch if you use P. Dichromate/Sulfuric Acid bleach.

Though it seems the problem when using Metabisulfite with Dichromate bleach has been reported as loss of image, and not fog.

I've not tried Dichromate with Metabisulfite, but Dichromate works perfectly with Sulfite. The problem could be with pH: Sulfite is alkaline, Metabisulfate & Bisulfate are acidic.