The app is heavy with using heart rate but those on beta blockers have restricted heart rate max. There is an option in Apple heath to take beta blockers into consideration and lower the expectation for vo2 max etc. For this app it could increase the sensitivity in the algorithm as well as slightly weight towards time. Ie while doing cardio the heart rate spike will be lower but that doesn’t necessarily mean intensity was lower so someone off beta blockers would spike to 110bpm while those on beta blockers would remain at around 90 but still elevated from baseline.