Mi Band 3 heart rate monitoring in 'Mi Band only' mode does not work properly

Mi Band Notify 6.9.2 PRO without Mi Fit
Mi Band 3 with fw 1.5.0.7
Samsung S8+ / Android 8.0

I have tried 5 and 10 min intervals and I have the same problem (after syncing with Mi Band):
https://1drv.ms/u/s!Ar4uqSZ68hV26kL08B-jyWqnIT9H

The intervals don't seem to respect the interval much, sometimes there are many many measurements during a small period of time with weird results: I am 100% sure that my BPM never exceeded 80 (I was sitting in front of my computer the whole time). Also, the band was in reach of the phone all the time (although that shouldn't matter in this mode anyway).

Basically, everything seems wrong with this mode. I have tried switching on/off/modes/intervals many times, but in 'Mi Band only', I always encounter these problems.

With 'Notify app only' mode, everything seems OK - maybe besides the fact that 2 measurements (sometimes more, if the two values differ, but that is by design, I guess) are always performed even with 'Improve results' mode off. I did not notice any difference in the number of measurements with 'Improve results' on or off.