When using a CS18 with cascaded RM mixers, it is very important to make sure the RM mixers are up and running and correctly cascaded as a single mixer before the CS18 is powered on and connected to the cascaded system.
When RM mixers are cascaed, they should be treated as a single "Logical Mixer" or "Mix System", and since the CS18 is actually connecting to this "Mix System" rather than to two individual mixers, this Mix System should be up and ready before you attempt to connect the CS18.
Failure to do so can produce unpredictable results, both in the CS18's AVB audio, and in the AVB audio between the two RM mixers in the cascaded system.