HTTP Health Monitor Failure Behavior
Try checking the Action on Service Down setting on the database server pool. By default, when a monitor marks a pool member down, no new or persistent connections are allowed to the pool member, however existing open connections are allowed to complete normally, assuming they can. For performance reasons, database systems often keep connections open (e.g. connection pooling). You can change the BIG-IP system's behavior when a monitor marks a pool member down by setting Action on Service Down to Reject or Drop. In brief, Reject resets the existing connection (TCP RST). Drop provides no notification to the pool member. See K15095: Overview of the Action On Service Down feature for more information.