Hi
I am a little bit confused with your posting
are you saying that you have one reth namely reth0
and one vlan of that is connected to another cluster ?
"specific reth0 (reth0.3622) which is connecting to another SRX Cluster."
one reth can only connect to one partner ( or partnercluster behaving like one node ) LACP checks on that thus preventing accidential misconnections to wrong nodes.
So use another reth interface for the second cluster
However if that is not the case there is another thing.
if you meant reth3622 --we had a problem on EX with ae numbers higher than around 500, as from 15.x onwards the numbers can bes used freely.
hope that can help you
regards
alexander