Due to a software defect introduced by  PR1432397 . When you configure a firewall with the forwarding-class feature on ACX2200, MX80, or MX104, this causes the "firewall process" - "dfwd" -  to restart unexpectedly. The result is no firewall is installed on the PFE. If you are using a firewall with "forwarding-class" on ACX2200, MX80, or MX104, do not upgrade to the following JUNOS software versions:

• 17.3R3-S5, or 17.3R3-S6
• ​18.1R3-S6 18.1R3-S7
• 18.2R3, 18.2R3-S1
• 18.4R2, 18.4R2-S1
• 19.2R1

This issue is tracked via PR1446556 .  Please check PR1446556 for detailed information.
As of 2020-06-01, the following Junos software versions have incorporated the fix:

• junos:17.3R3-S7 or later
• junos:17.4R3 or later
• junos:18.1R3-S8 or later
• junos:18.2R3-S2 or later
• junos:18.3R3 or later
• junos:18.4R2-S2 or later
• junos:18.4R3 or later
• junos:19.1R2 or later
• junos:19.2R2 or later
• junos:19.3R1 or later
• junos:19.4R1 or later
• junos:20.1R1 or later

Post to public on 2020-06-30