So it looks like LineageOS 15.1 for the Xiaomi Mi MIX 2 is now Treble supported, which is apparently leading to Magisk issues, at least for me…:

Archive: /sdcard/Magisk-v16.6.zip
inflating: META-INF/com/google/android/update-binary
inflating: META-INF/com/google/android/updater-script
inflating: arm/magiskboot
inflating: arm/magiskinit
inflating: chromeos/futility
inflating: chromeos/kernel.keyblock
inflating: chromeos/kernel_data_key.vbprivk
inflating: common/addon.d.sh
inflating: common/boot_patch.sh
inflating: common/magisk.apk
inflating: common/util_functions.sh
inflating: x86/magiskboot
inflating: x86/magiskinit
inflating: META-INF/MANIFEST.MF
inflating: META-INF/CERT.SF
inflating: META-INF/CERT.RSA
************************
* Magisk v16.6 Installer
************************
- Mounting /system, /vendor
mount: can't find /vendor in /etc/fstab
! Cannot mount /vendor
- Unmounting partitions
Updater process ended with ERROR: 1
I:Install took 4 second(s).
Error installing zip file '/sdcard/Magisk-v16.6.zip'

Hopefully there is a simple fix for this, but for right now, it looks like I’m stuck with root-less. XDA appears to be having issues as well, so I can’t look for my answer there. I guess that’s what I get for not reading changelogs before flashing ?

EDIT: 20180710-0900: So of course it was a simple fix… Just needed to update TWRP to the latest version, for me that’s: v.3.2.2-1.
I got some help from Jerry981028 on GitHub, who let me know what line supported recoveries need to have for Treble support: Android-Jerry/android_device_xiaomi_mido-twrp/twrp.fstab