The problem is that you’re using gradients.
Gradients are generally not linear, they are compressed at the ends, and wide in the middle. That is, they change from black to dark grey over a short span, from dark grey to light grey over a long span, then from light grey to white over a short span. When used as a depth map, this produces a curve.
To create a straight bevel, you need a linear depth map.
Personally, I would ditch the gradient fill entirely and blend between lines (Blend in Illustrator, Interopolate in Inkscape). If you’re stuck with using gradients, adjust your fill to account for the non-linear nature of the gradient.