Material Shades Differently in Preview than In Game

I have a cel shading material that uses a simple material parameter collection to create bands based on a forward vector from a directional light.

It looks as expected when previewing in the 3D viewport, with very clean light bands:

However, as soon as I play the game (or simulate the game), the shading has strange shadow artifacts (as seen on the left side of the face):

I don’t think it could be the material itself because that should the same rules regardless. (And I’d like to keep the material in a small group because it took a lot of hard work to make, so I’ll DM you if you want to see the material blueprint). I’m wondering if you all have any idea what it could be?

Maybe the material parameter collection?

Any help or direction is appreciated.