Hello -
What you are seeing is the SSAO generated by the actual mesh’s geometry, and as that geometry fills the screen it starts to “overwrite” the Normal information.
The good news is you can minimize this effect via adding a post process volume and lowering the intensity of the AO or play around with the Power and Bias settings to allow some AO while minimizing the overall Mesh contribution. As a side note, if you are using a material that is not white, the noticeability of the AO will be minimized as it is added to the Base Color, Roughness and Metallic Passes.
Hopefully that will get you back on track -
Eric Ketchum