I’ve noticed this on a few rides. Sometimes my bike does not have a shadow. My avatar’s shadow is there, apparently peddling away on an invisible bicycle.
I’ve also noticed that the scene’s overall brightness seems to change in large-ish jumps - sometimes several jumps per minute. This is something the Zwift app is doing - I’m not adjusting my monitor. The bike’s shadow’s appearance and disappearance seems tied to these shifts. A jump in scene brightness and the bike’s shadow disappears. A few seconds later another shift in brightness and the bike’s shadow re-appears.