fxp0 is not able to pass any traffic to any other interface on the router including loopback. This only allows destinations to itself it is a no transit interface.
this is confusing because the outbound traffic for this interface does use the root routing instance table inet.0 for route selection.
Starting in Junos 17.1 you can move the fxp0 to a special routing instance to isolate the routing table for this.