Hi,
I would review the FWSTrace.log to see if there is any error there that could help you identify why you are seeing a partial logout. There can be many reasons for this, one common reason is when the logout request has some mismatch, like UserID and NameID not matching in the Logout Request, or if the NameID format is the same in the partnershipand in the SLO request.
I hope it helps!