I trapped a piglin in a one by one section for a bartering farm. I then gave it a gold helmet with low durability from my mob farm because the tutorial I watched said that if you give a piglin gold armor, it will put it on and it shouldn't despawn. I was going to show my dad my completed bartering farm, but the piglin disappeared! But I still heard a piglin nearby, even though I could not see it. Though with the help of subtitles I was able to detect that the sound was coming from the one by one section I trapped the piglin in. When I broke a block to see what it was doing, an arrow came out of no where and shot me. The piglin was still there, but I couldn't see it or it's crossbow that it used to shoot me. Swinging my sword didn't do anything to it. Eventually I figured out that I could burn it with lava, which I did (rip piglin). I guess I'll have to use a name tag next time. (By the way I was playing on 1.16.4 because I did not my world to corrupt if I played on 1.16.5)

Ok update the piglin did the same thing even when I gave it a name tag. Maybe it was because he was standing on a hopper. I don't know whats going on. At least it still barters. 

Also yes I was using optifine