I think the explanation for this is hinted at in the user manual: " In order for allocation to work, the main product and the co-products of the multioutput process need to have the same flow property."
The manual is not clear about whether they all have to have the same *reference property* or if they all have to have any property in common. Most likely they all have to have the same property as the *reference property* of the *reference output* of the process. In any case, your other coproducts are probably measured in mass, and heat is not.