One more thing I thought about....
I use Photographer's formulary kind. With this particular kit, I make a stock solution of A and B where A is bleach and B is the actual sulfide toner. A is used full strength (not diluted) and used over-and-over where as B is diluted quite thin. I don't know what you are using but are you really supposed to dilute bleach part?
Also, with my toner, the B solution aggressively sucks air - to a point bottle caves in. I wonder if your's had a leak and oxidized?
With MGIV, it gets to dark chocolate brown (like you said) but I never saw "light and fluffy" brown tone. I use WT paper for that.