Villagers don't get scared of giants, and won't spawn an iron golem in defence.
How to reproduce:
Get some villagers (at least three, by spawneggs, commands or in the wild)
Summon a Giant with the command: /summon minecraft:giant ~ ~ ~ {Invulnerable:true} (this step of course near the villagers.
Note that the villagers will still refresh their trades and won't run away.
Environment
Java Windows Singleplayer world, no mods/datapacks.
Since snapshot 19w03a, Giants' AI was reverted back and villagers don't run away nor get infected by giants. So this hasn't been a bug since.