I have this same issue. From S.M.R.'s description, it sounds like this thread is the same issue. Judging by Garmin-Kevin's response in this other thread, @S.M.R. please post in the first linked thread about your issue. Garmin still has not fixed this.
Whoops; try again. I fixed the link. I'm collecting as much related complaints/issues and links to them because I have the same issue, and I want to provide enough info for someone at Garmin to have what they need to correctly identify/diagnose/resolve.
FWIW I have two data fields loaded and they seem to appear. I also did a full reset today (for another reason) and I use the method of backing up my device drive ad then copying back the sport profiles I have set up to the new device. Before I did this I reloaded the IQ fields from connect IQ and then they were added ok to the sport profiles I copied back in. They also record correct date on the .fit files relating those data fields.
I do find sometimes that when setting the fields up initially one sometimes cannot add them on the App , sometimes watch is better
