When $rollout.active(:use_new_search) is fully enabled, the Work.index_name function returns the name of the new index even on the Resque workers performing the indexing. This causes errors to pile up unless you enable stop_old_indexing. It also means nothing is getting reindexed on old search, making it difficult to switch back if things go kablooey.
Switch on use_new_search for all users, create a new work, and wait a few minutes for the work to be indexed. Then deactivate use_new_search and see if you can find the work in the old search. (Alternatively, check error logs.)