For DCI to work properly, it is mandatory to have logical full mesh (eBGP sessions) between Border leaf switches in all data centers.
This requirement has been explained in the following documents:
[1] EVPN-VXLAN Data Center Interconnect Design with Juniper Apstra—Juniper Validated Design Extension (JVDE)
"For seamless stitching, it is mandatory to have logical full mesh (eBGP sessions) between Border leaf switches in all data centers. This is to prevent issues during node or link failures on a primary border leaf which is the Designated Forwarder (DF), refer step 5 of the section EVPN-VXLAN Type 2 Seamless Stitching (Layer 2 only with MACSEC) Design."
https://www.juniper.net/documentation/us/en/software/jvd/jvd-datacenter-interconnect-design/results_summary_and_analysis.html
[2] Juniper Apstra User Guide -- Data Center Interconnect (DCI) / Remote EVPN Gateways
"There is also currently a full mesh requirement between EVPN gateways, although in future releases this requirement will be removed."
https://www.juniper.net/documentation/us/en/software/apstra6.0/apstra-user-guide/topics/topic-map/virtual-remote-evpn-gateways-staged.html
Example:
Go to BP > Staged > DCI > Integrated Interconnect > Local and Remote Gateways > Remote Gateways.Then edit the remote interconnect gateway and add 'bp1-dual-leaf1' and 'bp1-dual-leaf2', then update.
Note: