The earliest recollection of this bug is traced back to PAN-OS 8.1.12 - January 09, 2024. This bug is fixed in PAN-OS versions 8.1.12, 9.1.1. ( PA-5250, PA-5260, and PA-5280 firewalls with 100GB AOC cables only ) Fixed an issue where after you upgraded the first peer in a high availability (HA) configuration to a PAN-OS 9.0 release, the High Speed Chassis Interconnect (HSCI) port did not come up due to an FEC mismatch until after you finished upgrading the second peer. ( PA-5250, PA-5260, and PA-5280 firewalls with 100GB AOC cables only ) Fixed an issue where after you upgraded the first peer in a high availability (HA) configuration to PAN-OS 8.1.9-h4 or a later] release, the High Speed Chassis Interconnect (HSCI) port did not come up due to an FEC mismatch until after you finished upgrading the second peer. For more information: https://docs.paloaltonetworks.com/pan-os/8-1/pan-os-release-notes/pan-os-8-1-addressed-issues/pan-os-8-1-12-addressed-issues https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-release-notes/pan-os-9-1-addressed-issues/pan-os-9-1-1-addressed-issues