[Google Pixel 5a] naterz:
Having some issues getting it to install to ramdisk after updating to A12. Factory image'd both A and B slots (B being active) went through setup, installed magisk, installed fsociety kernel, booted TWRP and made a nandroid backup, then attempted to flash TWRP to the ramdisk via the advanced menu. This leaves me stuck at bootloader with a verified boot error. Now I do notice encryption is on for my device, but I have made sure to disable screen lock as is required (which allows me to decrypt data when fastboot booting twrp) - not sure if that matters at all. Seems like @eriklion is the only one who had a similar issue as me.

Figured it out after lots of messing around and research. Turns out in order to actually flash TWRP to A / B devices like the Pixels, you need to inject TWRP into BOTH boot_a and boot_b slots. This must have been taken care of on the A11 build and been an automatic process since it wasn't this much trouble for me on the October A11 image. Pretty sure my process went like this: Stock A12 Barbet - June build -> fastboot boot twrp.img -> TWRP/Advanced Menu/Install Recovery Ramdisk ->TWRP/Reboot Menu/Reboot To Recovery -> TWRP/Advanced Menu/Install Current TWRP -> TWRP/Advanced Menu/Install Kernel*** (I used the fsociety barbet A12 kernel) -> TWRP/Install Menu/Flash magisk.apk OR .zip ( magisk being completely optional I imagine) -> TWRP/Reboot Menu/Reboot to System.

The order of operations here might not be 100% correct towards the end of it, but it has to be pretty close. If anyone can verify the full process for A12 I would be super interested! Either way, I imagine how it functions now is about as good as it gets until the dev group irons out the rest of the A12 bugs and streamlines some features. @nijel8 Big time thank you for your work and commits to this device branch and for releasing a usable recovery build in the meantime. It's so cool to see the evolution of Android developer work on such a huge project like TWRP. If there's anything I can do for you regarding this device down the road, let me know. Happy to pull dmesg/logcat/whatever.

*** note: At one point prior to installing the Kernel / after TWRP was bootable via fastboot reboot recovery, I lost the touch screen driver. Ended up using a USB A->C adapter and a USB mouse to install the kernel through TWRP and rebooted. On next boot to TWRP the touch function was back. Somewhere along the way, I ended up with the vibrator/lag bug and had to disable vibration through TWRP settings to rid myself of it.

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Another note for @eriklion , I would suggest doing what you did - fastboot boot twrp.img - but then try sideloading the Rom OTA - and then, before letting the device reboot, load the twrp.img to ramdisk. Then perhaps reboot to recovery from within twrp, and if that works- then Advanced Menu - Install Current TWRP. Not saying it will work for you, but it's likely where I'd start if I were in your position. From what I've read, when you flash/load a systemOTA.zip on a Recovery-Via-Boot.Img A/B device, it will overwrite the existing boot.img and then switch to the other boot slot on next reboot.