oats as toppings in hotdog buns

i baked hotdog buns with oat bran as topping instead of the usual sesame seeds but it emits a foul smell after 3 -4 days and it has some mold formation after 6 days even though im using a bread preservative. is the oat bran the culprit. because i also bake hotdog buns before without any toppings and it is still good after a week.please help me.

thanks

jane