Monterey stuck in recovery loop after installation on internal drive only

I have a 2011 iMac 21.5 inch with upgraded metal Kepler GPU and SATA SSD. Running Big Sur with OCLP. I also have a Bootcamp partition with Windows. OCLP 1.5.0

I tried to upgrade it to Monterey and it's stuck in a boot loop after installation, going to recovery mode. Boot progress bar gets halfway then black screen and startup chime, takes me back to boot picker where I choose EFI boot, then immediately takes me into recovery mode. I did the upgrade from an installer on an external USB drive, and ran the installer from inside Big Sur. I also tried installing a fresh install of Monterey onto another volume on my internal drive, but got same boot loop.

However I have Monterey installed on an external USB drive and I can boot from that and it works perfectly. Monterey will boot from external drive but not internal drive.

I'm sure I'm choosing the right item in the boot picker. I tried removing the OC EFI, resetting PRAM, reinstalling Monterey, reinstalling OCLP, running disk first aid, but nothing is working. Why can I install Monterey to external drive but not internal??

The OCLP troubleshooting docs say "uninstall OpenCore and reset NVRAM" and "Machines with modified root volumes will also result in an infinite recovery loop until integrity is restored." What does this mean? How do I restore integrity?