Yesterday I was on my buddy's realm. He came there and had a pane of glass in hand. Then I noticed that you can look through the skin with this pane of glass. I could see everything behind him. We tested it on me too: I took the pane of glass in my hand and he could see through my skin.
This also works with armor racks in single player worlds by the sea.

Version: 1.16.220
OS: Apple iOS

PS: I can't go into the worlds of my friends either. Minecraft always hangs while looking for the world, and nothing reacts, so that I have to restart Minecraft.