Bubbles are and have to be formed due to cavitation effect from rapid cycling of the injectors opening and closing.
Part of the reason VW uses Solid hose is so that people don't try to diagnose issues that are not issues which are normal and not easily understood by those not familiar with injector operation.
You should see the air bubbles formed in a common rail injector at 30,000 psi...Looks like soda pop rushing thru at high pressure via the leak back (control circuit).
Nothing to see here... Literally.