Good advice! So the dark grains used are not similar.... roasted barley in the stout, small amount of black malt in the dubbel (a bunch of other specialty malts and dark sugar syrup tho). Both batches were dosed with gypsum, baking soda and calcium chloride.
Stout: gypsum 4.4 g, baking soda 4.4...
I have gotten this recently with a dry stout and a dubbel.... I ferment in a corny and transfer with co2 so I try hard to avoid oxygen... I am wondering also if it could be my perception of the dark grains although I don't seem to pick it up with commercial beers. It does seem to fade with time.