In 1.12, a villager will detect doors (house) up to 3 blocks above the ground level they are standing on as documented by the wiki: https://minecraft.gamepedia.com/Tutorials/Village_mechanics#Housing

This is used in many villager breeders to allow them to be turned off by moving the villager out of range of the doors. This is also used by many iron farms designs to enable the manipulation of villages and allow them to be stacked to increase the rates of these farms.

In 1.13-pre1, this detection range seems to have increase to 4 blocks. This means that many villager breeders will continue to produce villagers even though the player may assume their breeder is off which could be detrimental to their world due to the increased entities. This also breaks many iron farms that work just fine in 1.12 and will require rebuilding. This is a non-trivial task as many of these iron farms rely on precise positions of villagers at particular timings for them to work correctly. This change will mean a lot of reworking for technical players in 1.13. 

I believe this changes the census area from 32x32x8 to 33x33x9.

I've attached some screenshots from 1.12 and 1.13-pre1 showing the issue as well as a world download of each. Please note that when moving the villager out of range you need to wait for a little while for the village to deregister. I don't know the exact timing of that but if you wait 5 minutes that will be plenty.

I've also made a video showing the issue here: https://youtu.be/s-VzpcRLdJc

After posting the above video I was shown bug MC-123055 which maybe related but its marked as resolved currently.