"It is cold. We want to eat hot pot."
Although many people would immediately oppose, as a native speaker I agree with you that 很 is not mandatory in this sentence. Duo should accept it as correct. When the context provides enough information for the listener to conceive the magnitude of the adjective (it is as cold as one would think about eating hotpot), doing without 很 would not appear weird. On the other hand, for the same reason, with or without 很 does not really convey a different meaning.