Juniper SRX routing engines are sometimes vanished

CMK version:
2.0.0p18 Enterprise
OS version:

I recently added our SRX4600 cluster with snmpv3 and it auto discovered just fine. Almost immediately, the “cpu utilization node X routing engine” vanished for both nodes. I left them in there and they seem to come and go periodically… You can see here where one is detected and the other vanished.

Where should I start?