You must be new to AE. There is no stencil effect. There is Track Matte and if it stops working then you are either using the C4D renderer or the Ray-traced renderer and you did not read the warnings in the Composition Settings Advanced panel when you changed from Classic 3D.
Without complete details of your comp and your workflow it's impossible to tell what is wrong but that's my best guess.
To diagnose problems select all of the layers giving you problems, press the U key twice to reveal all modified properties, make sure the switches and the modes columns are both visible by selecting the second icon from the left at the bottom of the timeline, and just for good measure, make sure the comp is selected in the Project Panel so you can see the comp settings. Now start turning things off until you find the problem. If you can't figure it out then Print Screen and paste a screenshot of the project to the reply field on this forum. If you are on a Mac you can use the keyboard shortcut Shift + Ctrl/Cmnd + 3 to take a screenshot and then just drag it to the reply field from your desktop.
If you must use the C4D render engine to extrude the text then there are workarounds but we need to see the workflow and have details to show you which ones you should be using. Just try and remember that AE is layers and if you are having problems it is often because you are trying to do too many things on one layer.
I'm still not sure what you are trying to do. The Stencil Luma is not an effect, it's a blend mode and it creates transparency on all of the layers below based it on the luminance value of the layer with that blend mode. If you had 2D layers and you selected Stencil Luma for the blend mode you would knock out everything on the Background layer except where the text is and the opacity of the background filled text would be controlled by the luminance value of the text fill color. If you wanted to knock out the background completely so the text was creating a matte you would use Stencil Alpha. Then you would have full opacity in the text layer.
If you wanted to make a text sized hole in the background layer you would use Silhouette Alpha or Luma.
Changing the layer with the Stencil or Silhouette blend mode applied to a 3D changes the behavior when Classic 3D is used and blend modes are disabled using the other 3D rendering engines. Stencil Luma and Alpha don't work any more, Silhouette works differently. This is not a bug, it's a behavior and it's based on when the pixel values from the blend mode is calculated as the composite is being built. Neither of these approaches would be a good workflow to duplicate the Stranger Things open.
The end of the Stranger things open starts with a text layer that is filled with red:
The color of the text layer changes to gray or looses saturation which could be accomplished using an animated fill color or by a color correction effect like hue and saturation. Then the fill color fades out revealing the video below.
There are many ways to do this. One way to do it with just a text layer and a background video is to animate the text layer and the color change and then apply Set Matte from Effects>Channel to the pull the matte. Then just animate the opacity of the text layer to make it fade out. I would animate the Z position of the text layer and move it closer to the camera instead of animating scale. You'll get a more pleasing timing with a lot less work.
Another way would be to use a track matte but that would get a little more complicated if you wanted to fade out the text layer. There are no good ways to recreate this kind of transition with blend modes.
I'm also not sure what you are doing with the boy on layer 1 but the Set Matte or even Set Channels would be the easiest way to work the background.
So after the color changes how would you make the camera fly thru the text?
You and I made the position of the text layer in Z space until it gets close enough to the camera and properly position to fill the frame with video. If you fly past the camera the screen will go black.
Animating the camera is very basic After Effects stuff that you should know. If you are real new I strongly suggest that you spend a fair amount of time with the learning materials on the after effects product page.