This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
When a billboard actor passes across a seam in the scene capture cube's output its orientation changes. You can see this by moving the scene capture cube across the scene while watching the render target it's drawing to. Depending on the billboards location relative to the scene capture cube it will change orientation and a visible seam appears as this occurs.
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