I also just got a Pixel 8 Pro but haven't tried setting it up yet. I'm not surprised there are issues though, the Pixel 7 series had similar problems initially. I think there were both FordPass and Android updates needed to get it working.
I went through PAAK setup with the Pixel 8 Pro tonight on both the Lightning and Mach-E. I got the same message about limited device support, but then had no problems getting it set up on both vehicles. I put the fobs away and tested PAAK and had no problems unlocking/opening doors, and starting the vehicles. So the Pixel 8 Pro seems to be fine despite the warning message at the beginning.
One thing I made sure to do before starting was to delete the existing PAAK settings in Fordpass for my old device. Other than that, I would try the usual troubleshooting steps for Android apps (delete cache, etc). If that doesn't work you can reset all PAAK settings under general -> reset on the Sync screen in vehicle. That will delete all keys so if there are any other PAAK besides yours, those will also have to be set up again.