Here is what I have done:
1) Download the latest (5.4) magisk manager apk [Thread | Direct Link]
2) Set the update channel to both stable (14.0) and beta (14.3)
3) Choose install from stock boot img (with and without keep dm-verity and force encryption)
4) Copy the patched boot img from your phone:
adb pull /sdcard/MagiskManager/patched_boot.img .
fastboot flash boot patched_boot.img
1) With 14.0, the system boots up fine, but magisk isn't installed
2) With 14.3, I get bootloops, and have to reflash the stock boot.img in order to get it to boot.
Might be worth noting that the patched boot img is significantly smaller than the original one (about 25mb vs 40mb)