The bug

Using a lead on an iron golem changes their nature so that the will no longer attack hostiles, including a player that attacks them. Found this on an iron golem "farm" using a zombie; dragging 7 golems that were out of range so that they were within aggro and releasing them from the lead caused them to walk around in a somewhat glitchy way, with no apparent path finding (never really leaving the area they were dragged to), and not interested in chasing after the zombie. Lead two more zombies within range and the iron golems ignored them. I even attacked a golem myself and was not attacked.

Other iron golems in the village still act as normal. It only affects the iron golems that have been moved using leads.

To reproduce

1. Place a zombie inside of an iron golem trap
2. Summon an iron golem
3. Leash the iron golem
4. Unleash the iron golem
   → The iron golem will try to get to the zombie, but won't succeed, turn around sometimes, and it looks quite confused in general
5. Close and reopen the world
   → Now the iron golem is as angry onto the zombie as ever

Videos

Survival, village golems: https://youtu.be/sVl5fxA6GAY
Creative, self-built golems: Minecraft 1.15.2 - Singleplayer 2020-04-04 22-25-32.mp4

Code analysis

Code analysis by Chumbanotz can be found in this comment.