During lookaside eviction, updates that are committed but not yet stable are included in pages written to data files. If a subsequent checkpoint includes such a page, then updates could become durable before they are stable.
A test case for this would run with logging disabled, fix an oldest timestamp, then insert enough data to trigger lookaside eviction. Then take a checkpoint as of the pinned oldest timestamp. If the checkpoint contains any of the inserts then lookaside eviction has made changes durable too soon.