I followed your suggestion and rerendered in very low settings and got the same exact result. The strange part is that the window part is transparent on the psd, but when I saved it as a jpg it got a gradation. There is no information there except for the envsphere that is set emit light but not show in the rendered image. When I boosted the brightness the black section just started moving to the left and finally covered the whole image, no extra information there.

My render settings

Top View

Final render

It's the first time this happens, I usually have no trouble getting the depth map