Originally Posted by tysj
I want to say that it was relatively recently (maybe in the last 2-2.5 years) where if you only unlocked the bootloader, it would fail SafetyNet. For a while, you had to flash a kernel on top of unlocking the bootloader to pass SafetyNet.
My experience is if I unlock bootloader, I fail SafetyNet unless Magisk is installed. Sometimes after a reboot, I still fail until I load the Magisk app and have it check once. Then the phone is fine again.
As to the original question: I installed the newest beta of Magisk on my Pixel 3 XL, downloaded the factory boot.img from Google and patched it using the app. Flashing to my active boot slot caused fastboot to complain about no valid boot images. Flashing the original boot.img allowed the phone to start normally again.