3D Extrude Effect Imprefections and Artefacts

I think the answer in this case is that you're using a gradient fill, and that causes difficulties. The more complex the artwork the higher the likelihood of weird things happening in the 3D effects — I don't think these have been changed in decades, nor is it a focal point for heavy use among Illustrator users — and sometimes, probably not this time, a small change of rotation on one axis will fix an issue. You may find that it works better for you to apply the 3D effect to a path with a solid color fill, expand the appearance of the result, and apply the gradients to the fill of the expanded shapes.

Peter