Unless you are a multi-sport athlete married to Garmin Connect as your fitness hub, I suggest running EBC on your phone for indoor workouts. EBC has many more features and is easier to use. No Garmin issues to circumvent which can also pop-up after firmware updates. However, you shouldn’t need to downgrade the firmware unless everything was working fine before the last update. OTOH “weird” does come to mind when I think about my Garmin days. 
When working correctly with an Indoor profile configured on the Garmin, GPS Off, other gotchas mentioned by @ManofSteele addressed, and sensors paired under Garmin settings (not Xert player) then speed and distance are estimated based on power alone. This provides a rough idea of what you may have ridden outdoors on a calm day on a perfectly flat course. 