Small change to the snail mail letterbox

I love this letterbox, but it has a very annoying problem for me. When I play, I always have the walls halfway down, but as soon as the wall where the letterbox is is hidden, the letterbox is also gone. I have no idea why this was done.

I changed a small setting for the letterbox and now it remains visible even when the walls are hidden.

Small disadvantage, it is marked as CC, but you can always see it.