Here is the screenshot for proof because this is a random chance to have that weird texture and I am too lazy to have another of iron, but it’s gold dot on the drowned head and iron. It happen when a zombie with a golden, chainmail, and iron helmet submerged for a lot of seconds underwater. Also, it has an enchanted glint whenever the helmet is enchanted. It also occurred  with other helmet of other types. It can happen when any drowned were helmet. Oops I mean that it occurs when the drown equips a helmet.