ECMP per-flow load balancing can be configured in SRX devices using the following documentation:
https://www.juniper.net/documentation/us/en/software/junos/flow-packet-processing/topics/topic-map/security-ecmp-flow-based-forwarding.html
In reference to:
The device is running pre-21.4R3.
When configured:
set policy-options policy-statement load-balancing-policy then load-balance per-packet
set routing-options forwarding-table export load-balancing-policy
And, two static next-hops are configured for the same network and one of the next-hop is a PPoE interface. Traffic stops forwarding.
If only the PPoE static route is deleted, traffic starts flowing. If only the PPoE route is left, traffic starts flowing. If both static routes are non-PPoE, traffic starts flowing.
In summary, the trigger conditions and symptoms are:
Running pre-21.4R3.
Configured load-balancing.
Configured 2 static routes to the same destination.
At least one of the routes is through a PPoE interface.
Traffic to the destination stops being forwarded.
The issue is known to resolve in 21.4R3 and onward.
The issue is known to affect 20.2R3-S2.
It is unknown if the issue affects current supported releases 21.2, 21.4R1 21.4R2. If you upgrade to a current supported release (non-EOE): https://support.juniper.net/support/eol/software/junos/ and the issue persists, please open a JTAC case.