CM Exposure for world AO and mat override, what's happening?

What’s up in 2019-05-20 version (maybe earlier as well, not sure)? I have a very simple indoor scene with a default diffuse override material. World AO is enabled with 0.1 facrot and 1m distance. Happens with both sRGB and Filmic.

At exposure 0, everything is normal.
At exposure -1, the rendering starts out very dark, then some brightness is added.
At exposure +1, the rendering starts out a little bright, then brought back as the rendering finishes.

Compositing is empty, so the adjustment doesn’t happen there.

So my question is, where does this adjustment take place, and why? There must be something wrong with my scene (simplified attached), but I can’t figure out where…
WeirdShadingChangeOnCompletion.blend (707.1 KB)