When both "add-path multipath" and  "advertise-inactive" are configured under BGP, and a BGP route is inactive, multipath flag is reset and multipath is not formed. 
As a result, "add-path multipath" will not process these inactive routes to a peer with "add-path" capability. 
Consequently, inactive BGP routes are not advertised to that peer, despite the "advertise-inactive" setting. 
This issue is seen only with peers that support "add-path" functionality, below configuration can cause this issue:

set protocols bgp advertise-inactive 
set protocols bgp group XXX neighbor X.X.X.X family inet unicast add-path send multipath 
set protocols bgp group XXX neighbor X.X.X.X family inet6 unicast add-path send multipath 

Below CLI command can be used to monitor the issue: 

user@device> show route x.x.x.x 

user@device> show route advertising-protocol bgp <neighbor-address>

The following symptom can be noticed with this issue:

1. BGP Inactive prefixes will not be advertised to the BGP peer.

This is a software defect and addressed.

For the WA and other, please refer

https://prsearch.juniper.net/PR1861799