I had a SolrCloud cluster where every node is behind a simple round-robin load balancer.
This cluster had two collections (A, B), and the slices of each were partitioned such that one collection (A) used two thirds of the nodes, and the other collection (B) used the remaining third of the nodes.
I observed that every request for collection B that the load balancer sent to a node with (only) slices for collection A got proxied to one specific node hosting a slice for collection B. This node started running pretty hot, for obvious reasons.
This meant that one specific node was handling the fan-out for slightly more than two-thirds of the requests against collection B.