That picture just made me realize something - is that booster pump tied into the plumbing before the sand filter?

If so, then the booster pump that feeds the Polaris is using unfiltered water from the skimmer/main-drains. Therefore, if junk gets past the pump's skimmer basket, then it can easily get sucked into the booster pump and returned to the pool through the Polaris port. That would explain why you have debris coming out of your Polaris return and not any other return. That would also then be a VERY GOOD reason to install a shut-off valve or isolation valve on the pressure side line as you are diverting pool water away from sand filter and right back into the pool. It's a smaller diameter line and there's a booster pump in the way so the flow is going to be small but it still would exist. Some booster pumps don't allow water to pass through them when they are not ON but others do, it depends on the make and model. A check valve with a high spring force after the booster pump would help cut down on the flow of water sneaking past your sand filter.

Of course, if my view of your plumbing diagram is wrong, then ignore the above.