I doubt you are doing anything wrong. I suspect it has much to do with your asthma and allergies.
While 90bpm is the lowest I’ve heard of for this type of workout I would forgo the HR. Clearly you are working as hard as you able. HR is not a great indicator of intensity when the muscular load is very high like in weighted pack carries. If you read the articles on this site or some of the forum posts regarding this type of muscular endurance training you will read repeated references to not using HR as any sort of guide to intensity in these workouts.
The determination of intensity should come from the perceived exertion and local leg muscle fatigue.
I hope this helps,