I had set up a couple default bike settings for 'Mountain bike ride' and for 'E-Mountain Bike Ride'. After getting a new bike and swapping that bike out for the previous one as the default bike for 'E-Mountain Bike Ride', many of my activities that had the previous default bike associated with them had the bike removed and the drop down to select a bike appeared on those rides. It seems there is a bug with loosing the associated bike on activities when you change your default bike for a given activity.