It's a common issue, NVIDIA webdrivers for GTX Titan are known to mess with the MacPro5,1 BootROM's NVRAM volume, saving a big blob inside the VSS stores that blocks AMD GPUs to work. If you can't access the Apple native BootPicker with your RX 6800XT MacVidCards flashed GPU, you have this issue.
You can try a deep NVRAM reset, if doesn't work, you will need a BootROM reconstruction service. You will also gonna need to clean install Monterey, since when you install an unsupported release via OCLP, the root patching process removes all GPU drivers but the drivers for the GPU you had installed when you did the macOS install.
So, if you changed the Titan for the RX 6800XT without re-installing Monterey, the 6800XT didn't work because of the missing GPU drivers.