Samsung Galaxy Tab S6 Lite screen turning off in protective case

I have a new Samsung Galaxy Tab S6 Lite with a pen. Also I have a protective book-like case (Aceline Streak STR-008).
The device works fine, except one thing: when I open the case and fold it backwards (I mean flip the front cover backwards behind the back side of the device) – screen gets turned off. It doesn’t happen if I just put the tablet onto something. Device doesn’t reboot or has any kind of abnormalities with this. When I remove the front surface of the case and let it just hang down freely – the screen turns on.

What can be the reason of this behaviour?