• Happened on Fabric with Sodium + Iris installed

This is the underlying problem of Rethinking Voxels, which will be equally present on Fabric, (Neo)Forge or even OptiFine.
These shaders don't render their voxels off-screen, otherwise your performance has dropped, roughly speaking, by another 4 times, or even more.
Moreover, there will be a similar problem with all texture bloom effects.

Most shaders only use screen-space effects, meaning anything off-screen is not taken into account such as reflections, auto exposure, DOF and bloom as mentioned before. I've tried multiple different methods of taking cubic screenshots and all of them suffered from this issue.

Complementary Shaders work fine when taking panorama. The only issue can be too much bloom on the edges of the cubic sides, so you should decrease it.