Update:
I dont need new glassed (phew).
I did not miss this when scanning 'resolved issues' at each new release.
I have been checking v12.x Rx versions since I am on v12.
The V12.1 release notes page makes no mention of fixing the PR in question under the R4 resolved issues.
Taking a look in v11.4 release notes under it's R6 resolved issues I see what I had been looking for:
Flow & Processing:
"On all high-end SRX Series devices, when the ingress and egress interfaces were located (or active, in case of reth interfaces) on different nodes of a chassis cluster, filter-based forwarding (FBF) did not work correctly. [PR793057: This issue has been resolved.]"
It would seem that someone forgot to put this in the v12.1 R4 release notes update (Might want to update this Juniper).
Case closed.