Hi,
Is you flickering very noticable, or just patchy from denoising? I had a separate issue where Tangent space normal doesn’t work with instanced foliage and path tracing. It took me ages to figure out, but if you go into the view mode where you can just see the Normal for everything you can see what is happening with the flickering. For some reason it doesn’t affect Ray tracing though, only path tracing.
I ended up turning off tangent space normal in my foliage master materials, and that fixed the problem, but you can also convert placed foliage actors into a static mesh, and that fixed the problem leaving tangent space normals on, but it was not viable with the large amount of foliage I was using in my scene.