Shadow Catcher not working if all object are "Shadow Catcher"

Pretty simple issue here, but not able to find someone talking about it.

I have a really simple scene.

4 concentric rings rotating with 1 light..

I only want shadow. Nothing more. Everything is working perfectly if I only tick 3 objects with "Shadow Catcher". But as soon as all objects have "Shadow Catcher" ON, then the render stop rendering any shadow… I only get a full transparent render.

Any clue or workaround ?

Thank you in advance.