The potential workaround, which doesn't seem to be solid, is to sideload the OTA, then immediately enter fastboot, and flash /vbmeta and /boot. Sometimes you'll still end up in Rescue Party, where you can either flash back to stock and try again, or give up and wipe. We are still testing over at the Pixel 5 forum.
Thanks V. I am running this test today:
1) P6P as shipped
2) Setup w/o network so it won't update
3) Download latest OTA image
4) Patch boot and sideload OTA
5) After OTA, boot directly into bootloader
6) Flash verity with flags
7) Flash patched boot
9) See what happens
If this works would show that, at least this time, we can sideload an OTA and get perm boot w/o wiping.
In another thread, I was going to run some tests w/ P3XL and P4XL but things got busy. Did you still need me to try anything? It seems like pre P4a root is now well understood and the focus is on >P4a. Let me know if you need something.