I have customized the data screens for the "Bike Indoor" activity to my liking. However, when I start a Bike Indoor activity by selecting a smart trainer workout, it shows a different set of data screens. I can't see a way to edit these screens.
Another person had the same issue on Forerunner 955 here: Smart trainer data screen - Forerunner 955 Series - Running/Multisport - Garmin Forums