Some of the methods in ConcurrentUpdateSolrClient accept an aditional collection parameter, some of this methods are: add(String collection, SolrInputDocument doc) and request(SolrRequest, String collection).
This collection parameter is being ignored in this cases but works for others like commit(String collection).
elyograg noted that:
Looking into how an update request actually gets added to the background
queue in ConcurrentUpdateSolrClient, it appears that the "collection"
information is ignored before the request is added to the queue.
From the source, when a commit is issued or the UpdateParams.WAIT_SEARCHER is set in the request params the collection parameter is used, otherwise the request UpdateRequest req is queued without any regarding of the collection.