Fit Contributor fails to write developer fields to MESG_TYPE_SESSION or MSG_TYPE_LAP messages.

Connect IQ Fit Contributor creates  'developer_data_id' and 'field_description' messages for  MESG_TYPE_RECORD messages in the Activity FIT file, but it fails to create them with MESG_TYPE_SESSION or MESG_TYPE_LAP .

Code - adapted from Module: Toybox::FitContributor

 if ($.session == null)
        	{
            $.session = Record.createSession({ :name => "raceQs", :sport => Record.SPORT_GENERIC});
             $.session.start();
	bananasEarnedField  = $.session.createField(
	"bananas_earned",
	   5,
	   Toybox.FitContributor.DATA_TYPE_UINT16,
	     {:mesgType=>Toybox.FitContributor.MESG_TYPE_SESSION, :units=>"B"}
	        ); 
        }
        totalBananas = 100;
        bananasEarnedField.setData(totalBananas);
    

Compile and run an app containing the code in the SDK simulator or a physical device (I used a Fenix5)

Save the Activity to a local file.

Run FitCSVTool.jar from the Fit SDK package on the resulting fit file

Expected result:

To find In the Fit File contains 

Actual result

No developer_data_id or field_description Definition or Data messages in the output.

Further info

1. When I modify the code to generate a MSG_TYPE_RECORD, the expected fields are generated and the developer data is found in the 'record' messages
2. when I run with MSG_TYPE_LAP no developer_data_id or field_description Definition or Data messages in the output.
3. I have built a Fit file decoding program in PHP to examine session, lap and record messages and to decode developer data by interpreting the developer_data_id and field_description messages. This confirms what I'm finding from FitCSVTool.

[UPDATE]

I have since discovered that when writing a developer field to a MSG_TYPE_LAP message, the data is  in fact written to the message, but there are no developer_data_id or field_description messages which explains why FitCSVTool fails to detect the value.