The issue appears to that CheckLedgeDirection almost always returns true, which returns the leftward side step. This is correct for the leftward direction, but is not valid for the rightward direction and prevents movement.
In CheckLedgeDirection, the check performed is uses the complete capsule. One workaround suggested is to reduce the capsule radius by the amount specified in PerchRadiusThreshold to better align the two, but there is probably a better solution.
In a project with a character movement component driving a pawn (such as the third person template), set CanWalkOffLedges to false and attempt to walk of a ledge while angled to the right. The pawn will stop moving.
When angled to the left, the pawn will be able to walk along the edge as if using SlideAlongSurfaces.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-200815 in the post.