When I tried to put double chests very quickly, it happened that one of them was doubled but cut in half.
I made the game crash with F3 + C to have all the data. The crash report is attached to this post.
I forgot to say that it's just a visual error, I rejoined to my world and it's a single chest.
I am unable to reproduce this issue in 21w17a. I placed down multiple chests without being able to reproduce this.