Try taking a look at this post
Apparently, cubemap seams come from out of sync post processing values.
I found out that because when the CaptureSource of USceneCaptureComponentCube is set to ESceneCaptureSource::SCS_SceneColorSceneDepth, the alpha channel of the paramcube returns the z-axis depth of each of the 6 faces of the cube, so there is a seam, and I convert the depth to radial distance and the seam is eliminated!
