This is an effect of using a double sided material. Normally a polygon will only be visible from one side, the other side will not be seen. This is good as it means the program only needs to draw the side that will be visible to the user. For example, if you have a box you only need to draw the outside as the user will not be able to see the inside.
Sometimes though you want the polygon to be visible from both sides. For example, glass on a window, the lenses on sunglasses, grass on the ground or leaves on a tree. Basically anything that uses some sort of transparency and can be seen from both sides.
For reasons best known only to the developers at Zwift the riders and bikes use double sided materials which is very wasteful in my opinion as it means each bike and rider is effectively drawn twice.
Source: Makes 3D objects for games and is ever so slightly triggered when I see sloppy model making.