If you're fan viewing a mass-start event that started before you logged into the game, and the event has a jersey set to override the participants' individual jerseys, you won't see them wearing the jersey set for the event.
If you're participating in the event or you log into the game before the event starts, you will see the correct jersey.
This is a bug, and we're working on a fix in an upcoming update.
If you're participating in the event and want to be seen in the event jersey by all riders who are fan viewing, you can set your avatar to wear the event's jersey in your customization screen (if you have the jersey unlocked on your account). See our article on how to customize your avatar.