I don't think it's bacterial as I noticed it in a commercial brew and someone else did, as well, in what was assuredly a different batch. I had another thought last night after I posted, and that's that 'sesame flavor' comes from the oils in the sesame seeds. I'm thinking it could potentially be a combination/interaction of the flavor compounds from the darker grains plus some oil from the hops. Although hop oils are generally said to contribute 'floral', or 'citrusy', or 'spicy' flavors, perhaps in some combination and/or processing with grains it is possible to wind up with a 'sesame' type flavor.