If it's that class specifically, you might need to unregister and re-register it as an autoloaded node.
Here's the general explanation: to give you autocompletion for a scene, Godot needs to parse it completely, and to read or compile all the code in there. It won't do it for closed scenes because they might be big or complex to parse, slowing down the program while that happens.
So by default, I believe that Godot will only provide you completion for the scenes you opened manually, one way or another. It could be: