[Xiaomi Mi Mix 3 Guides, News, & Discussion] PWn3R:
I did not, I just tried to boot the device and it kept rebooting. I'll try that later flash that patched boot, then flash TWRP then flash Magisk.

Edit: [MENTION=4544860]osm0sis[/MENTION] - here's a pastebin of the boot logcat before the bowl of bootloops kicks off. https://pastebin.com/uFZm3CXh - I'm sifting through it, but I've never looked at one before (if my kernel didn't work in my builds, I just hijacked someone else's LOL).

Ok, so I see this in the working one:

03-16 20:22:22.893 560 560 W auditd : type=2000 audit(0.0:1): initialized
03-16 20:22:24.269 560 560 I auditd : type=1403 audit(0.0:2): policy loaded auid=4294967295 ses=4294967295
03-16 20:22:24.269 560 560 W auditd : type=1404 audit(0.0:3): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295
03-16 20:22:25.389 522 522 W init : type=1400 audit(0.0:4): avc: denied { setattr } for name="mi-camera-vendor2hal" dev="tmpfs" ino=15637 scontext=u:r:vendor_init:s0 tcontext=u:object_r:tmpfs:s0 tclass=fifo_file permissive=0
03-16 20:22:25.389 522 522 W init : type=1400 audit(0.0:5): avc: denied { setattr } for name="mi-camera-hal2vendor" dev="tmpfs" ino=15638 scontext=u:r:vendor_init:s0 tcontext=u:object_r:tmpfs:s0 tclass=fifo_file permissive=0
03-16 20:22:25.389 522 522 W init : type=1400 audit(0.0:6): avc: denied { setattr } for name="mi-camera-vendor2hal" dev="tmpfs" ino=15637 scontext=u:r:vendor_init:s0 tcontext=u:object_r:tmpfs:s0 tclass=fifo_file permissive=0
03-16 20:22:25.389 522 522 W init : type=1400 audit(0.0:7): avc: denied { setattr } for name="mi-camera-hal2vendor" dev="tmpfs" ino=15638 scontext=u:r:vendor_init:s0 tcontext=u:object_r:tmpfs:s0 tclass=fifo_file permissive=0
03-16 20:22:26.988 563 563 I SELinux : SELinux: Loaded service_contexts from:
03-16 20:22:26.988 563 563 I SELinux : /vendor/etc/selinux/vndservice_contexts
03-16 20:22:26.989 557 557 W BootMonitorThre: type=1400 audit(0.0:8): avc: denied { getattr } for path="/sys/fs/pstore/console-ramoops-0" dev="pstore" ino=17673 scontext=u:r:mqsasd:s0 tcontext=u:object_r:pstorefs:s0 tclass=file permissive=0
03-16 20:22:26.990 561 561 I SELinux : SELinux: Loaded service_contexts from:
03-16 20:22:26.991 561 561 I SELinux : /system/etc/selinux/plat_service_contexts
03-16 20:22:26.996 557 557 I mqsasd : Starting mqsasd
03-16 20:22:26.997 557 569 D mqsasd : last boot result: 2
03-16 20:22:26.998 557 569 D : delete file: /cache/mqsas/perseus_V10.0.11.0.PEEMIFH_last.zip
03-16 20:22:26.999 1 1 W init : type=1400 audit(0.0:9): avc: denied { setattr } for name="last_kmsg" dev="proc" ino=4026532129 scontext=u:r:init:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0
03-16 20:22:26.999 1 1 W init : type=1400 audit(0.0:10): avc: denied { setattr } for name="last_kmsg" dev="proc" ino=4026532129 scontext=u:r:init:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0
03-16 20:22:26.999 572 572 W cat : type=1400 audit(0.0:11): avc: denied { read } for name="last_kmsg" dev="proc" ino=4026532129 scontext=u:r:mqsasd:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0
03-16 20:22:26.999 557 569 D : The next cmd: cat /proc/last_kmsg
03-16 20:22:27.008 557 569 D : The next cmd: logcat -L,-b,all
03-16 20:22:27.009 573 573 W logcat : type=1400 audit(0.0:12): avc: denied { read } for name="pmsg-ramoops-0" dev="pstore" ino=17674 scontext=u:r:mqsasd:s0 tcontext=u:object_r:pstorefs:s0 tclass=file permissive=0
03-16 20:22:27.015 562 562 I hwservicemanager: hwservicemanager is ready now.

And this is the one that's broken:

03-16 19:59:41.787 550 550 W auditd : type=2000 audit(0.0:1): initialized
03-16 19:59:43.669 550 550 I auditd : type=1403 audit(0.0:2): policy loaded auid=4294967295 ses=4294967295
03-16 19:59:43.669 550 550 W auditd : type=1404 audit(0.0:3): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295
03-16 19:59:43.929 517 517 W init : type=1400 audit(0.0:4): avc: denied { setattr } for name="dsp" dev="rootfs" ino=23568 scontext=u:r:vendor_init:s0 tcontext=u:object_r:rootfs:s0 tclass=lnk_file permissive=0
03-16 19:59:43.929 517 517 W init : type=1400 audit(0.0:5): avc: denied { setattr } for name="firmware" dev="rootfs" ino=23597 scontext=u:r:vendor_init:s0 tcontext=u:object_r:rootfs:s0 tclass=lnk_file permissive=0
03-16 19:59:43.929 517 517 W init : type=1400 audit(0.0:6): avc: denied { setattr } for name="bt_firmware" dev="rootfs" ino=23581 scontext=u:r:vendor_init:s0 tcontext=u:object_r:rootfs:s0 tclass=lnk_file permissive=0
03-16 19:59:43.929 517 517 W init : type=1400 audit(0.0:7): avc: denied { write } for name="/" dev="rootfs" ino=1 scontext=u:r:vendor_init:s0 tcontext=u:object_r:rootfs:s0 tclass=dir permissive=0
03-16 19:59:44.809 517 517 W init : type=1400 audit(0.0:8): avc: denied { write } for name="storage" dev="rootfs" ino=23590 scontext=u:r:vendor_init:s0 tcontext=u:object_r:rootfs:s0 tclass=dir permissive=0
03-16 19:59:44.809 517 517 W init : type=1400 audit(0.0:9): avc: denied { setattr } for name="mi-camera-vendor2hal" dev="tmpfs" ino=23647 scontext=u:r:vendor_init:s0 tcontext=u:object_r:tmpfs:s0 tclass=fifo_file permissive=0
03-16 19:59:44.809 517 517 W init : type=1400 audit(0.0:10): avc: denied { setattr } for name="mi-camera-hal2vendor" dev="tmpfs" ino=23648 scontext=u:r:vendor_init:s0 tcontext=u:object_r:tmpfs:s0 tclass=fifo_file permissive=0
03-16 19:59:45.009 547 547 W BootMonitorThre: type=1400 audit(0.0:13): avc: denied { getattr } for path="/sys/fs/pstore/console-ramoops-0" dev="pstore" ino=23087 scontext=u:r:mqsasd:s0 tcontext=u:object_r:pstorefs:s0 tclass=file permissive=0
03-16 19:59:45.020 553 553 I SELinux : SELinux: Loaded service_contexts from:
03-16 19:59:45.020 553 553 I SELinux : /vendor/etc/selinux/vndservice_contexts
03-16 19:59:45.023 547 547 I mqsasd : Starting mqsasd
03-16 19:59:45.026 551 551 I SELinux : SELinux: Loaded service_contexts from:
03-16 19:59:45.027 551 551 I SELinux : /system/etc/selinux/plat_service_contexts
03-16 19:59:45.028 547 555 D mqsasd : last boot result: 1
03-16 19:59:45.029 560 560 W cat : type=1400 audit(0.0:14): avc: denied { read } for name="last_kmsg" dev="proc" ino=4026532129 scontext=u:r:mqsasd:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0
03-16 19:59:45.029 1 1 W init : type=1400 audit(0.0:15): avc: denied { setattr } for name="last_kmsg" dev="proc" ino=4026532129 scontext=u:r:init:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0
03-16 19:59:45.029 1 1 W init : type=1400 audit(0.0:16): avc: denied { setattr } for name="last_kmsg" dev="proc" ino=4026532129 scontext=u:r:init:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0
03-16 19:59:45.029 547 555 D : delete file: /cache/mqsas/perseus_V10.0.11.0.PEEMIFH_last.zip
03-16 19:59:45.031 547 555 D : The next cmd: cat /proc/last_kmsg
03-16 19:59:45.041 547 555 D : The next cmd: logcat -L,-b,all
03-16 19:59:45.053 547 555 E : filepath=/cache/mqsas/perseus_V10.0.11.0.PEEMIFH_last,zip_dir=/cache/mqsas/,zip_file=/cache/mqsas/perseus_V10.0.11.0.PEEMIFH_last.zip,params=-o,-9,-j,/cache/mqsas/perseus_V10.0.11.0.PEEMIFH_last.zip,/cache/mqsas/perseus_V10.0.11.0.PEEMIFH_last
03-16 19:59:45.053 552 552 I hwservicemanager: hwservicemanager is ready now.

I think it may not be mounting everything that needs to get mounted?