I'm not the multivalve expert but there are 3 things that come to mind:
1) The Gasket (Called a Spider Gasket because it looks like a web) was installed incorrectly and still is.
2) The spring(s) for the multivalve are either damaged or installed incorrectly.
3) One of the chamber walls in the Multivalve is damaged, cracked or holed, and the water's getting through there into the backwash port.

There could be other issues, but, again, I'm not the multivalve expert.

Carl