Rubbished may be overstating it, I don't recall exactly. And it sounds like you and I installed Magisk differently. I installed the apk normally on the phone like any other app, then downloaded the boot image corresponding to the current version of LOS, then had the Magisk app patch that boot image, then used a PC with adb to fastboot flash the patched image back to the pro1x.
If anyone else here did Magisk that way, then to preserve root with OTA updates of LOS, I recommend doing the "interrupted OTA and install to inactive slot" trick that I mentioned above. It saves a bunch of hassle.