for a while ago I used to get the well-known bug when I sync the device with Garmin Express which is getting Connect IQ datafields do not show up and got replaced by Timer field. Then when I try to do it set it up again but when I try to insert a new Connect data field it shows 2/2 and tells me that I need to remove one first to insert a new one. The problem is only that I don't have one selected! I used to avoid getting into this trouble by not using Garmin Express and rely on the Mobile App.
But recently I started getting this problem during the Run App only when I start the activity (a Workout from Training Calendar), so all the Connect IQ datafields got replaced by a Timer field but luckily the counter Connect IQ (0/2) resets, so I could re-add the connect IQ data fields while on the activity which is painfull. but when I check the run app outside any active activities I find that all settings in the app are correct, Connect IQ fields are selected!
I do not know if this is related to the Mobile app ? or just another bug from Garmin firmware ?