"Open next lesson in the collection" not working

Yesterday I remarked that, by clicking on “Open the next lesson in the collection” (of how it’s phrased in English…), it’s not the following lesson that’s open, but a random lesson among the following ones. This happened with the “Anne of Green Gables” collection, but Jolanda told me she had the same problem with alsuvi’s Spanish collection.
What’s the problem?