If the Wikipedia article can be trusted (always a question, particularly given the lack of inline citation), the imagery predates Christian ascendancy (to Hadrian, at least). This would mean that it is conceivable that even if knowledge of the topology of the world was later lost, use of this imagery persisted, shorn of its original meaning.
I would note that the two footnotes I checked in this post did not reveal sources that asserted “that church authorities taught (and enforced) a belief in a flat earth”, but that both these sources had their claims subtly exaggerated in the retelling – both times to state that the sources were impugning “the Church” as an institution, when they had not done so.