Please send me (david@intervals.icu) one of those files from the device. Maybe I can spot something.
Unfortunately workout fit files generated by Intervals.icu are not the same as workouts uploaded to the Garmin API for sync to a device. The Garmin API requires a JSON document and not a fit file. They generate the file for the device from that.
Intervals.icu supplies the “description” field for each step to the API. The API docs do not mention any other fields.