routing: fix mc blinded path behaviour.

Because we hot swap the target public key when sending to a
blinded path we need to penalize all nodes after the introduction
point.

routing/result_interpretation.go

@@ -517,11 +517,15 @@ func (i *interpretedResult) processPaymentOutcomeIntermediate(route *mcRoute,
 		if introIdx == len(route.hops.Val)-1 {
 			i.finalFailureReason = &reasonError
 		} else {
-			// If there are other hops between the recipient and
-			// introduction node, then we just penalize the last
-			// hop in the blinded route to minimize the storage of
-			// results for ephemeral keys.
-			i.failPairBalance(route, len(route.hops.Val)-1)
+			// Penalize all hops after the introduction node up to
+			// the final recipient. We cannot just penalize the
+			// last hop because when sending a paymenet we hot swap
+			// the target pubkey for a temporary one, so without
+			// penalizing all hops because the last hop does not
+			// really have an effect on our pathfinding.
+			i.failPairRange(
+				route, introIdx, len(route.hops.Val)-1,
+			)
 		}
 
 	// In all other cases, we penalize the reporting node. These are all