This seems to happen mainly with legacy kit such as old Nortel routers and the like. The peering won’t come up at all despite everything looking OK config wise.
When doing a bit more digging, the following message can be seen in the Juniper logs:
bgp_recv_open: peer x.x.x.x (External AS 12345): received NOTIFICATION code 2 (Open Message Error) subcode 4 (unsupported optional parameter)
set protocols bgp group VENDORA neighbor 192.168.250.2 disable-4byte-as
This sets the peering to 2 byte AS number support only and allows the peering to come up.