Step on render with opaque material

Hi I wanted to ask for a problem I have on an automotive rendering.

I’m trying to recreate a studio light to illuminate a machine to be rendered.
The point is that if I use an opaque material it creates steps on rendering only if I use artificial lights (rectangular panels with neon material) … if I use HDRI maps instead, why doesn’t it happen?

The obj model is perfectly exported and I used Bevel = 0 attenuation = 0.5

opaque material + artificial lights → steps in rendering why ??
shiny material + artificial lights → no problem (perhaps due to the shiny material that hides the steps)
opaque material + Hdri → no problem