I've had RQ done. It's respitory quotient. It is the ratio of CO2 exhaled to oxygen consumed. Fat burning produces a lower RQ than carbohydrate because of the different chemical makeup of fats vs carbs (put simply: carbs carry oxygen with them). So for the same energy output you require less oxygen on carbs than on fat and the RQ changes.
I've seen fat burning peak at around 70% at 50% vo2max. It dropped to zero as I crossed AT, which at that time was around 270W.
I've also seen it peak as low as 25% when detrained.
My max HR was 175 on day 1 and 155 on day 2. Absolute max I can achieve is >190.
Good luck in getting to bottom of this.