M400 losing stride sensor calibration factor

Hello everyone!

I've had an M400 for about 8 years. Recently I've noticed that the manual calibration factor on my stride sensor changes (not to 1.000, but to another value) periodically, without my changing it. I have the stride sensor calibration set to "Manual." I think, but am not sure, that it happens after I sync with Polar Flow using my phone. I don't see any settings in Polar Flow where the stride sensor calibration factor would be set.

Anyone else have this problem, or know what I am doing wrong?