Debris coming back into pool

[SunnyOptimism]

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.

[swimdaddy]

I think you nailed it. With the booster pump on the pressure side unfiltered water water could make its way through and back into your pool. Quick question: could the booster pump be on the suction side of the main pump?

[SunnyOptimism]

Quick question: could the booster pump be on the suction side of the main pump?

No, you should not plumb it that way. Two pumps running in parallel on the suction side of the line would "fight" with each other for water. I don't know the specs of that booster pump but it would likely lose the fight with the pool pump. Also, booster pumps are designed to boost the pressure and flow rate of water already moving in them, they are not really designed to initiate flow on their own. There are other PF users way more knowledgeable about pumps and water flow than me so perhaps they can chime in.

Also, the setup as it is depicted is not wrong per se, it's just missing a critical component - a way to isolate the Polaris line when not in use. It's probably ok to use "dirty" water to run the Polaris since any large size debris will just get captured by the Polaris leaf bag. You could hook the booster pump in on the return side (clean side) of the filter but then you'd have lower pressure water to work with and that might not make the Polaris happy.

Simplest fix, in my opinion, is to add an isolation valve just after the PVC T-connector on the vertical leg so that the Polaris port can be shut off when not in use. This should result in slightly higher filter pressure and no more "junk" coming out of the Polaris port when not in use.