Hello Peter,
The difference may come from several things. First, I would check the output size is the same between Substance 3D Designer and Blender. I would also check that the maps are on the same bit level (as you already did), but most of all, Blender is using the CPU to render the SBSAR which is maybe not the case for Substance 3D Designer.
To check and switch which engine is working on Substance Designer:
Tools > Switch engine… (or the F9 key).
SSE2 will be the CPU and Direct3D the GPU. There might be a slight difference between the CPU render and the GPU render. If this difference looks like the issue in blender, you can simply adapt the graph with the SSE2 engine on.
If the problem persists, it probably comes from the integration in blender.
Keep me up to date and have a nice day,