Hello,
I have seen something interesting regarding this topic. BGP l3vpn routes stored in bgp.l3vpn.0 can have a different route age than the routes in their respective "routing-instance".inet.0 table. It seems that when there is a change in the next hop, like an LSP having a change in route, the route age gets refreshed in "routing-instance".inet.0 table, but the corresponding route in bgp.l3vpn.0 does not refresh it's route age.
This behavior would not correspond to an update message from the BGP neighbor.