I have water leaking out of the waste port on the multiport valve even though it is set to filter. What is the cause?

Inside the valve there is a 5 spoke gasket (spider gasket) that creates a seal between the various settings. This seal may have become damaged or worn over time and needs to be replaced, the gasket has come out of its seat due to the valve being turned incorrectly, or there is debris inside the valve preventing a seal.