For what it is worth, I have the same / similar experience with REM detection in Sleep as Android with a Xiaomi Mi Band 2 for heart rate monitoring versus the REM detection of my Fitbit Alta HR.
For details, see my earlier thread ‘Does using heart-rate really improve REM detection?’ http://forum.urbandroid.org/t/does-using-heart-rate-really-improve-rem-detection/1781