I had problems with a little setup of mine. I created a screen (func_button) pointing on a func_camera with a button on the side creating explosions with a countdown shown using env_text.
The explosions were too far away in deathmatch mode (only) and weren't displaying (only the decals were showing through the camera).
I created a 2x2 underground tunnel from the remote location to the target location and it worked! Explosions are now seen through the screen.
I guess it's a matter of vis leaves threshold. Things happen up to a certain vis leaf offset.