It's probably because the client gave you bad private feedback (the kind of feedback you can't see and which affect the JSS), even though the client gave you 5 stars.
In other words, the client wasn't happy.
The private feedback metric does NOT update after every contract so the feedback that let it siink to that very low level may or may not have been the last contract that closed and may or may not already be calculated into the JSS
JSS is so complicated. In addition to the common ones we all know about that affects the JSS, they also have windows months. I till this day don't fully get how it works. Last time I check, this is what I got.