Vapor/bubbles after pump

Hi everyone,
Got a question I'm trying to solve here. It is for an experiment I'm running at achool.

I got a pipe system with a centrifugal pump. There is cavitation in front of the pump due to the pressure falling below the water Vapor pressure. When the water flow enters to the backwards fan centrifugal pump, the Vapor/bubbles should collapse due to the high pressure at that location. But there are a few bubbles that passes through the pump. Why is this the case and what is happening?

Thanks for viewing!