The Liferay DXP Cluster documentation contains an incorrect/misleading statement:
If you are using multicast to handle your cluster, add this property to portal-ext.properties:
This is not correct as both multicast and unicast clustering needs this property to be set.
Also, it seems that the whole article on dev.liferay.com is more up-to-date somehow.
Could you please check this as well? https://dev.liferay.com/en/discover/deployment/-/knowledge_base/7-0/liferay-clustering