Explanation/Reference:
When a BGP router reports itself as the next hop, whether because of an explicit neighbor next-hop-self configuration or implicitly as a result of participating in an EBGP session, BGP allocates a new in label and adds an entry to the MPLS forwarding table, creating a label-to-next-hop mapping.
Note: When a BGP router does not report itself as the next hop, whether because of an explicit neighbor next-hop-unchanged configuration or implicitly as a result of a participating in an IBGP session, BGP does not allocate a new in label. Instead, if the route is advertised as a labeled route, BGP uses the existing out label. This feature is used mainly on route reflectors.
References: https://www.juniper.net/techpubs/en_US/junose16.1/topics/concept/mbgp-bgp-next-hop-self- overview.html