The bug:

It is possible to see entities like players, items or mobs through walls because of their blue lines with hitbox (F3 + B).

Steps to reproduce:

1. Turn on hitboxes (F3 + B) and get any entity like a mob or a player to face a 1 block deep wall.
2. You can tell from the other side of the wall that there is an entity through the wall by looking at the blue line from their hitbox.

Intended behavior:

You shouldn't be able to tell if there is an entity like a player through walls.

Actual behavior:

You're able to tell if an entity like a player is through a wall or not by turning on hitboxes (F3 + B)