FitContributor Data Not Appearing in Garmin Connect - Can't Figure Out How To Debug

I'm working on a DataField that I've recently released, focused on walking activities, and I've added a feature to add activity step counting to FIT data. I'm fairly confident I've got the implementation right because I've looked at examples of how others have done it - and followed along (monkey c, monkey do ) - my app runs on the simulator and on my fenix 5x watch just fine, no crashes and I've attached the debugger to see that the correct data is being sent with a call to myField.setData(value) on every compute cycle. However, I just went for a test walk using my DataField and there's no sign of my activity step count in the Garmin Connect mobile app or Garmin Connect website.

I'm not really sure where to go from there - I've pulled the .fit file from the watch but I don't know how to open it - it isn't a plain text format, and I tried opening it in Monkeygraph but that didn't work (that app just seems to be flat broken for me - you select "Open FIT file", point at the file and press okay, and nothing happens - same with a .fit file exported from the simulator).

I'd loaded the app onto my watch with the Build For Device wizard - do I have to push it through the app store as a BETA for FIT contributions to work? Ideally I'd like to be able to test it in the simulator too.