Mishaal Rahman reported Magisk works on the Pixel 3 XL running A12 DP1.Android 12 Pixel 5 magisk not working. after flash the patched boot.img reboot to bootloader.
I guess there may be issues to iron out with other (later) Pixel models, but your best chance of compatibility will be using latest Canary build (not beta/stable) due to recent fixes included. You haven't indicated what build you tried.
@MishaalRahman, did you use Canary Magisk build? Thanks, PW