Ok, here are the logs I could pull. I installed the njh47f stock image to my Pixel XL, booted, went through the initial setup and everything, then turned it off, went into the the bootloader and fastboot booted TWRP (3.2.1.0) and installed Magisk 15.3 (1531) (although this happens 14.6, 15.0, 15.1, 15.2, and every other build of 15.3) and it installed totally fine.
I rebooted the phone and started logcat so it would log right from boot, and that log is attached in the zip as "startup.log". I took a look in it and it does appear as though Magisk starts to some degree, but as I stated in other posts, once the phone boots, Magisk Manager is not installed. I installed Magisk Manager 5.5.5 myself, and then started logcat again, launched Magisk Manager and let it log (attached in the zip as "magiskmanager.log") for about a minute while Magisk Manager just hung doing nothing.
As for the actual Magisk log that is supposed to be located in /data/adb/magisk, well it never gets created for me. I checked through the TWRP file manager and while the /data/adb/magisk folder does exist, there is no log file in it.