Unfortunately this is not working for me.
After flashing the patched boot img and rebooting, Magisk manager still shows Magisk is not installed, and Titanium backup still reports that it can't get root.
I noticed that in the example posted by Craig, fastboot flash boot magisk_patched.img wrote to boot_b by default, whereas for me it's always boot_a, so I explicity flashed boot_b for good measure. I've done this about 5 times now....
I started with the boot.img from GoogleDrive, the patching in Magisk Manager seemted to go fine, and I notice that my patched version is exactly the