The server versions need to be the same on all nodes participating in the cluster. We don't support and neither indent to support clusters with different server versions.
You can handle this to some extent by writing custom Externalizers for your custom session attributes.
Your externalizer code will need to handle serialization compatibility.
Externalizers are contributed via service loader (e.g. /META-INF/services/org.wildfly.clustering.marshalling.Externalizer)