it may be that your heart rate falls to 50 or below. that it is flagged as a Abd rhythm as that’s a medical guide less than 50 is bad (in general for the genpop) but fi your fit and a good athlete in rest that can happen often. it is for me every nigh 1.5-3 hours of HR 44-50 and they get flagged at other check your hr if it is around 50 or less that is why health authorities make these guidelines and I know Fourth Frontier is working on modifying the algo to accommodate this. but this is where med guidelines for average JOe and a good athlete with much lower breathing rate and HR conflict. there are also questions of liability. know our own condition for me below 50 is frequent. this may be the reason but check with a FF specialist arrhythmia advisor or your cardiologist.
if it is this and you are in good shape with good BP and a wake resting HR between 50-58 youre actually doing well not something to worry about