Okay, I was able to reproduce your problem. Unfortunately, it seems that you can’t use Lumen with this setup. If I understand correctly, you’re using a post-process-based toon shader, which means Unreal has to constantly recalculate how all your shaders should look. The issue is that Lumen also continuously calculates lighting, and the two systems don’t work well together.
You’ll need to disable Lumen. To do this, go to Project Settings, then Rendering, search for “Dynamic Global Illumination Method”, and set it to None.
If you really want to use Lumen, I’d suggest changing your approach and using a material-based toon shader instead.