With the new 1.14 village mechanics, the old, classic iron farms no longer work. As of 1.14.3-pre1, Mojang started an attempt to re-introduce the possibility to create fully automatic iron farms again (https://twitter.com/MiaLem_n/status/1135562763827384321). In this first pre-release, all that was required were one zombie and three panicking villagers, allowing people to create extremely efficient iron farms with relatively little effort (see iron flood, etc.). As of 1.14.3-pre2, the mechanics were nerved so that villagers no longer can remain in panic mode indefinitely. They now have to sleep and work from time to time to continue spawning iron golems, making the farms a little more complex (not a bad thing as long as it works). The community came up with several different solutions to make this possible.
I tested a few different designs in a creative world, which all seemed to stop working after some time, so I ended up trying this one: https://www.youtube.com/watch?v=z_lSLw1gWkc. Turns out, this one has the same issue. It works flawlessly as long as you stand directly in front of the farm and don’t move very far, but as soon as you start doing other things in the world (flying around, building things, always in render distance to the iron farm), the villagers will stop spawning golems after one day/night cycle. It’s impossible to reset the farm after this, the only way to make it work again is to kill all villagers, add new ones and wait 1-2 day/night cycles. I suppose that’s not how this mechanic is intended to work, so I’m reporting this as a bug here. Other people seem to have the same problem (see comments in the video or https://www.reddit.com/r/Minecraft/comments/byjol0/iron_farm_1143_pre_2_problem/). I doubt there’s anything fundamentally wrong with these iron farm designs either, especially considering they work just fine for hours when standing still in front of them. Would be great if the devs could make this mechanic more reliable
A good workaround I found for MC-27535, another issue that causes the iron farms to stop working, is by using 6 named zombies instead of one. None of them disappeared so far, while single zombies are usually gone after a while. Still not as bad as the problem described above, zombies are way easier to replace than villagers.