Observe that there is a Post-Process Volume, and it is in a Data Layer in the starting
map. It is the only thing in the Data Layer.
Play PIE. Observe that the Post-Process Volume disappears from the Outliner, but still
exists in the world. Entering it with the camera has a visible difference from its
post-processing material
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-190951 in the post.