BUG: Mirroring not working with MadVR

When using MadVR as Video Renderer, mirroring (Alt + Numpad 6) seems to be not working. Vertical mirroring (Alt + Numpad 2) works however.
When switching to another Video Renderer mirroring works again, so the problem seems to be somehow connected to MadVR.

Can someone confirm and/or give a solution?

MadVR does not support mirroring.

So why does vertical mirroring work?

Because it support flipping.

Hmmmmmm.
I don't really get it: What's the technical difference between mirroring and flipping? Why would MadVR support the one but not the other? If you can implement the one feature, how hard can it be to implement the second one as well?
I'm honestly confused!

It is simply a fact and given the development state of MadVR, don't expect it to be added.