Possible sprite miss-match, all standard golems' arms are missing icons.
Only standard golems affected, humanoids are intact.

because some other mods changed the golem model, and my mod use the vanilla golem model. The only way to fix this is to remove that mod who changed vanilla golem model

Problem identified: Fresh Animations changed vanilla iron golem model. Remove that mod to fix this visual bug

Yeah, thx, that is exactly the issue.

To those who stumbles on this, removing the entire resource pack (Fresh Animations or whatever changes the iron_golem model) is unnecessary.

Just delete all iron_golem folders/files from resource pack, thus nullifying the overrides to iron_golem.