This issue occurs with the Additive and Alpha Holdout settings.
Attached the repro project.
HDR can be enabled by adding "r.allowhdr=1" to the engine's ConsoleVariables.ini.
For reproduction projects, you can change the HDR settings with the 1 and 2 keys.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-140417 in the post.