I’ve just stumbled upon this weird “bug”, I don’t know how I didn’t notice it sooner.
The 3d model of the wall is from Quixel, no displacement, just a normal map.
IR - Instance : correct result
IR - Render Instance : weird shading
Prod - Render Instance : weird shading, like a blend between both results.
Another example
Corona 12.2 hotfix
Cinema 4d 2025.2.1