My old post for this same question got way too long. Since this still is a hassle for me I will post a new question, but try to keep it really short and simple.
1). Shiny metalic materials with textures often have very distorted normals / reflections in RPR.
2). The exact same model, untouched, has none of this issue when rendered using Cycles.
3). Sometimes changing topology in the area will fix, or improve the issue.(althogh sometimes the topology needs to be where it is.)
4). I don't understand why I would have to alter topology to make it work right since Cycles obviously can do it right this way.
5). I want to add that this version of RPR (2.0.162) is now VERY stable on my machine, compared to earlier versions. So you are really doing a great job with every update!
This first video shows this object, shiny distressed steel, rendered first in ProRender, and then simply switching over to Cycles, and ALL the black distorted reflections are gone.
The second video shows how simply going into edit mode and dissolving the surface edge left over from a mirror modifier, and letting it solve from a corner to corner instead, gets ProRender to render that single flat surface the way it should be.
Here's the blend file. I don't know if it will open right (with all the material textures etc. But I just want to provide all I can to help to get an answer to this.
Thank you all very much!