Unable to flash on Poco F3 due to inability to mount '/mnt/vendor/spunvm/'

[cyangalaxy]

Poco F3, Unlocked Bootloader, Android 11, MIUI 12.5 Stock ROM.

This is likely a problem with my phone, somehow. Or the version of TWRP for my particular device. However I'd still like to kindly ask for help, as I've no idea where else to ask.
I've asked on Reddit and on the Xiaomi.eu Forums, but those haven't received much traction.

Basically, I can't mount '/mnt/vendor/spunvm/' in TWRP, and neither can Disable_Dm-Verity_ForceEncrypt, or any other Force-Encrypt.zip. All Force-Encrypt ZIPs fail.

I also can't manually edit /vendor/etc/fstab.qcom, as the filesystem /vendor/ is read-only, and in both ADB Shell and TWRP Terminal I'm unable to set it read-write (Invalid Argument). However, mounting /vendor read-only works.

I'm also not sure if '/mnt/vendor/spunvm/' and '/vendor' are related.

I've asked in the Subreddits r/AndroidQuestions, r/LinuxQuestions and the Xiaomi.eu Forum:
https://www.reddit.com/r/AndroidQuestions/comments/o2b6lc/problems_trying_to_disable_android_11s/
https://www.reddit.com/r/linuxquestions/comments/o29q3v/devblockdm3_is_readonly_when_trying_to_remount/
https://xiaomi.eu/community/threads/how-to-disable-force-encryption-on-poco-f3.61635/

TWRP Recovery.log (with noticable mount problem) recovery (1).txt
TWRP Screenshot: https://i.imgur.com/KQkPqNp.png

I would appreciate any tip/solution/follow-up question/reply regarding my problem. This isn't really considered an issue, because I just want to have Encryption disabled, but I've been trying to do this for more than a week, it's been a bit frustrating and I really don't want to give this up.

Things I've tried:
Using Xiaomi.eu Custom ROM, Flashing Force-Encrypt.zip before/after Magisk, Not using Magisk at all, Re-flashing Stock ROM and flashing Force-Encrypt and/or Magisk directly after Flash (before the first Android Boot), manually editing fstab.qcom (Can't paste modified fstab), Setting /vendor as read-write (doesn't work, Invalid Argument)

Edit: Have put Screenshot on Imgur instead, as Screenshot was taking up the whole page.