I have FR965 with Android Garmin IQ connect. First I selected high - during activity, then I selected low - general use, I went back to check during activity,the setting is set to low same as general use.
Did you try setting it directly on the watch? Does it work there, or also not?
It works on the watch but not by the app.
I recommend reporting it to Garmin through https://support.garmin.com