using a cookie that is essential for non-essential purposes, is not allowed. So using a load-balancer cookie is fine, as long as it's only used for load balancing.

Once it's used for other (technically non-essential) needs as well, one needs to find another basis for processing or ask permission for that second purpose(consent basis).

Also, if the LB cookie can be non-identifying, while fullfilling the stated technical purpose, it must not allow identifying users. So for LB cookies, one must not use a unique ID per user, but an LB ID instead. Something like "node1", "node2" etc...