Most of my activities tracked with a Garmin Epix 2 are missing the PM serial. I checked with Fit File Viewer and the PM serial is here.
However playing around with the custom fields the device type is 2 (not the expected 11).
typedef FITUInt8 FITAntplusDeviceType;
#define FITAntplusDeviceTypeBikePower((FITAntplusDeviceType)11)
typedef FITUInt8 FITBleDeviceType;
#define FITBleDeviceTypeBikePower((FITBleDeviceType)2)
The reason is that you’re connected to the PM as BLE and not ANT+