Topology: See the attachment.
According to the design, the return traffic to S6502A should go through NE80A---NE40---S6502A, but the actual route is NE80A---NE80B---M20---S6502A.
Change the priority of black hole route of NE40 to 12, solving the problem.
Both NE40 and M20 advertise black hole route, but M20 is the first to advertise it which is learned by NE40 through ISIS. However, the priority of black hole route imported into NE40 locally is 60, less than 15 which is learned through ISIS. So the route is not activated, and cannot be imported into LSDB of ISIS.
When the network consists of equipment from different vendors, pay attention to the principle for parameter setting of routing protocol.