I guess a small amount would stay in the water (nothing in chemistry is ever completed to a true 100%), but as long as the sorbent is magnetized itself (as in it doesn’t rely on attaching to the PFAS to become magnetic) it should be collected by the magnet whether it manages to bond to a PFAS or not. You might be able to leave an extremely trivial amount in the water by running it through enough magnets.

Edit: I forgot why I wrote this in the context of the comment chain. Even if the sorbent bonds to the wrong thing, it should still be pulled out by the magnets (minus whatever small amount gets left behind). Assuming the sorbent comes in magnetized and does not rely on the PFAS to become magnetized.