Well, I'm out of ideas, my normal maps end up looking trashy all the time.
I have this one gun for which I'm trying to bake normal map:
Using details from higher detail version of the low poly mesh (not all the details are done yet because even these few, can't bake properly):
But after baking in Blender 2.83.1 results look like this:
I've made sure to keep these 2 meshes in the same spot, even made a cage mesh but here are always some seams and these 2 squares are, for some reason, being stretched to the right. I've tried baking normal map from high poly mesh in Substance Painter but, no difference. I don't know anymore, for some reason I just can't bake a damn normal map. Here's the .blend file so you can look inside by yourself. Hopefully someone else could find out what's going on there.