In 1.12.2, doors that were in the detection range of a villager were added in xyz order.  In 1.13, the order has changed.  Existing iron farms will need to be rotated 90 degrees and mirrored to function again (in other words: completely torn down and rebuilt).  This could seriously discourage upgrading to 1.13, and serves no conceivable purpose.  This issue is distinct from, though related to, MC-131029