Addition from Garmin Product Support:
This report is for when you are having difficulty getting more than 2 CIQ data fields added to any single Activity Profile (without power cycling the watch). This occurs when you editing the data fields (DFs) and data screens (DSs) using Real Time Settings in the Garmin Connect mobile app or on the watch itself. If choose more than 2 DFs on any one DS, the 3rd DF will not be accepted in the app. When chosen on the watch itself, the 3rd CIQ DF shows an error on the screen and then reverts to timer. <End of Garmin comment>
The only way to make it stick is to change it when you're starting a workout, otherwise if you change it for later it usually gets lost.
Also the Android app never syncs the right fields - might put in a separate post.
This happens for running and walking at least and sometimes it picks random other fields from the ConnectIQ list of fields. Grrrr!