Hi CreativeRaul,
The effect that you notice, while switching the light on and off, is based on the “Default Light”.
If no light is in the scene, the “Default Light” is switched automatically to on, so one can see something. This is, BTW, changeable in the Render-Setting>Options>Default Light
You will find also in the Editor View an option for that, but only to change the directions (Editor View>Options>Default Light.)
As you have excluded the objects from the back-light (Project List), they lost not only this “Default Light”, hence they looked different, and they lost as well as the other light-source.
Remedy: Place a point light close to the camera, a little bit “camera-left” and up. This should give you the effect back.
TIP: To get only a Lens Effect, you can go to the (background light-object is active) Attribute Manager>General, there you can switch off Diffuse, Specular and GI (as you like of course). The Lens Effect is provided from the position of the light, based on some parameters of course, such as Intensity. In your case you had only a few objects (basically) to move, but this way works faster. ;o) Note: lens-flare sits on top of everything, light glare or visible light sources (based on position) behind or in-between, etc, but it needs an light wrap/light glow to really sit in the picture, which is a post process.
Suggestion: If possible, render your scene with and Object Buffer for all objects in front of the light and then switch them off and render the Lens Flare etc, it gives you a better freedom and the default light is not affected as well.
Multi-Pass comes to mind, but in this case, I would render two sessions.
Please let me know if that works for you.
My best wishes for the project.