NLSR

Seems like for HR, it is not straight forward to remove the FIB entry completely in a short time.
Example:
c---a---b---d:

Once successfully converged, the routing table at a will have the following routes:
Routing Table:
Destination: /ndn/b-site/%C1.Router/cs/b
NextHop(Uri: udp4://10.0.0.2:6363, Cost: 0)
NextHop(Uri: udp4://10.0.0.6:6363, Cost: 1.41926)
Destination: /ndn/c-site/%C1.Router/cs/c
NextHop(Uri: udp4://10.0.0.6:6363, Cost: 0)
NextHop(Uri: udp4://10.0.0.2:6363, Cost: 1.41926)
Destination: /ndn/d-site/%C1.Router/cs/d
NextHop(Uri: udp4://10.0.0.2:6363, Cost: 0.935617)
NextHop(Uri: udp4://10.0.0.6:6363, Cost: 2)

FIB at a will contain the following prefixes:
/ndn/c-site/c nexthops={faceid=266 (cost=0), faceid=264 (cost=1419)}
/ndn/d-site/d nexthops={faceid=264 (cost=936), faceid=266 (cost=2000)}
/ndn/b-site/b nexthops={faceid=264 (cost=0), faceid=266 (cost=1419)}

After NLSR on b goes down, a will have the following routes - trying to route through c to b:
Routing Table:
Destination: /ndn/c-site/%C1.Router/cs/c
NextHop(Uri: udp4://10.0.0.6:6363, Cost: 0)
Destination: /ndn/d-site/%C1.Router/cs/d
NextHop(Uri: udp4://10.0.0.6:6363, Cost: 2)
Destination: /ndn/b-site/%C1.Router/cs/b
NextHop(Uri: udp4://10.0.0.6:6363, Cost: 1.41926)

And FIB:
/ndn/c-site/c nexthops={faceid=266 (cost=0)}
/ndn/d-site/d nexthops={faceid=266 (cost=2000)}
/ndn/b-site/b nexthops={faceid=266 (cost=1419)}

HR at a does not know that c is routing to b through a itself
(In LS, a will update its adjacency LSA and c will calculate accordingly).
So in the case of HR, for the route to be completely gone NLSR must wait for the expiration of Coordinate LSA (30 minutes by default).

(Note that on Router d the route will be completely gone in a short time).