It is best that cluster members share (1) management layer networks and (2) all layer 2 user networks.
If two cluster members are layer 3 connected, when access points fail from one cluster member to another, their clients will receive a deauth, which defeats the purpose of a cluster, entirely.
I am convinced that a "layer 3 cluster" is a true feature, rather than just telling if you if have your cluster misconfigured. If you want fast redundant failover, clustering where two MDs are physically in the same location and share layer 2 management vlan as well as layer 2 user vlans is the best configuration. Anything else is not intended