In my hardware, an AMD Ryzen laptop with dedicated Nvidia GTX 1650 card, Reverse PRIME is kind of not working but working at the same time.
I already had an Xorg configuration file for Prime Offloading (AMD integrated GPU used by default) and that was working well, the issue I have with Reverse PRIME is that I get a black screen when I connect an external monitor, it may seem like it’s not detecting the HDMI monitor but that would be a false assumption, there is actually a video signal being sent but I think the framebuffer is not being copied at all. I know that’s the issue because when I go into Nvidia settings and change the gamma and brightness options the display actually goes through different tones of white while I change the settings.
This problem is not exclusive to the proprietary Nvidia driver though, if I do the same with the Nouveau driver there is also a black screen but in Nouveau the cursor also shows up. I heard that this might be an issue with dma_buf in the amdgpu driver (it was supposedly fixed on Intel) and with Nouveau I got a kernel stack trace when connecting the HDMI cable so I’m going to report this to the Linux DRI bug report infrastructure.