I may be wrong here, but from some initial reading, and listening to early podcast EPs, it doesn’t seem as though heart rate is used much at all in calculating fitness trends on Xert.
If I’m doing a session that’s quite low intensity, I’m not likely to budge MPA, and certainly not generate a ‘breakthrough’, but I’m likely getting fitter.
For one, over time, I’ll likely be able to hold a higher wattage at the same heart rate - an indicator of increased fitness, and arguably a ‘breakthrough’.
Is it because heart rate can fluctuate day to day because of heat, dehydration and illness? If so, have you looked at ways at smoothing out these? E.g. resting heart rate test each morning?
Part of the reason I feel strongly about this is that Xert doesn’t seem to ‘reward’ LSD rides for example - which are incredibly valuable. And, IMO this promotes a culture of NO INTENSITY NO GAIN which I think is detrimental to the sport as a whole, and so often leads to burnout.