Aknown population of a heat resistant spoilage microorganism is added to a food product. what might be going on?

The expired date of the product should become shorter.
Cooking will expose the food to a high temperature which will kill most of the microorganism that lives in it. The reduced number of microorganism will need more time to reproduce, thus the expired date to become longer.
Since the microorganism added is heat resistant, it wont be killed by the cooking process and their number is still the same. This will allow them to spoil the food faster.