some stuff just doesnt render while you are looking through a player who is in spectator mode or just yourself in F5. stuff like chests, players, water, signs, chests, mobs, player heads, item frames, beacon beams, shulker boxes, glass blocks and glass pains, ender chests, banners, armor stands, paintings, books inside lecterns, slime & honey blocks, ice, and entities just dont render. you can see right through them. there are probably many more things you can look through but these are just some of the examples i found

to reproduce just go into spectator mode press F5 and hover above these blocks and they will be see-thru. they should still appear under your half transparent head like normal regular blocks but these things dont