[OnePlus 3T Questions & Answers] gpago:
So, quick explanation. Pie update showed up on my OP3T and I have an unlocked bootloader, TWRP 3.3.0-1 and Magisk 18.1 on OOS 5.0.8 (Oreo). Additionally I had a second user account and a Work Profile on my main account.

After reading on the decryption issues (and against my better judgement), I decided that maybe it could work through System Updates since it detected root and was going to install the full package. Big mistake. Got a "Decryption unsuccessful" and sadness.

I had of course taken a full TWRP backup to be safe.

I'm going to put the detailed explanation below (hidden) because it's quite long.

Now in more details. Along with the TWRP back up, I had an oandbackup -backup- of all apps (main and secondary account, but not work) and manually copied all internal storage files on my PC. After the "Decryption unsuccessful" message, I pressed the "Reset Phone" button (probably a stupid decision) thinking I'm good since I have a backup. Phone rebooted, with TWRP replaced (as expected), and I'm in the new OS. Played around a bit in Pie [added my account, registered a fingerprint and PIN, connected to WiFi and checked a few apps] and then I thought, "ok, time to go back", and then it happened.

I rebooted the device and it asked for password to boot (which was the PIN I added in the OS), and then rebooted again in Fastboot to flash TWRP again in order to restore the backup.

TWRP flashed successfully and I copied over my backup. Restored System, Data and Boot, wiped Dalvik & Cache, rebooted, shows the Oneplus logo, gets stuck for a bit and then goes to Fastboot. Ok...probably messed up somewhere, let's try again. Same thing.

I guess the notion that I had the TWRP backup made me feel safe, because I continued to experiment.

After a full 24hours awake trying to restore my phone to its previous state, I have the following:

Restoring System, Data and Boot results in the device booting and showing the Oneplus logo and then after a few seconds, going to Fastboot.

Restoring EFS, Recovery and/or Cache makes no apparent difference.

Restoring Data and Boot, but System Image instead of System makes the device boot and start a lengthy process (possibly of encrypting the entire Data partition) lasting about 2hours, during which the device becomes hot enough (~45C / ~113F) that holding it for more than a couple of seconds is very uncomfortable. After it finishes the lockscreen has my wallpaper, app notifications (like VPN) show up and the second account is there, however my old PINs for either accounts don't work and the Data partition can no longer be accessed in TWRP

Data successfully decrypted, new block device: '/dev/block/dm-0'
Updating partition details...
...done
[COLOR="red"]Unable to mount storage[/COLOR]
Successfully decrypted with default password.
Updating partition details...
...done
[COLOR="red"]Unable to mount storage
Failed to mount '/data' (No such file or directory)[/COLOR]
Full SELinux support is present.
[COLOR="Red"]Unable to mount /data/media/TWRP/.twrps[/COLOR]
MTP Enabled

When Data is inaccessible in TWRP, only formatting it can bring it back (empty, of course). And then it needs to be formated to F2FS again as well (Oneplus uses F2FS for the Data partition).

Flashing the Oxygen OS 5.0.8 zip file after restoring System works the same as restoring with System Image.

The closest I've come to restoring my device to how it was before is either:

Restoring System, Data and Boot , in which case booting ends up in Fastboot.

OR

Restoring Data and Boot, and System Image, and then not being able to login (plus, no access to Data from TWRP).

I have literally no idea what else to do. If anyone has any idea or suggestion, it would be greatly appreciated.

I'd rather wait for some suggestions before trying to flash other zip files that deal with encryption, since I never had to do that when I first rooted with Magisk.

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How the device is supposed to be:

Bootloader unlocked

TWRP Recovery 3.3.0-1

OxygenOS 5.0.8 (Oreo)

Magisk 18.1

Main user account

Secondary user account

Work Profile (with Shelter) on Main user account

What I have available:

Full TWRP backup of all partitions [Data, System, Cache. System Image, EFS, Recovery, Boot]

oandbackup backups of Main & Secondary user accounts (apps and APKs)

Manual file backup of Main user account's Internal Storage

All OxygenOS Oreo zip files that Oneplus was releasing over time

TWRP images (3.2.1-0 and up)

Stock OxygenOS recovery (they used to have it available)

Magisk Installer/Uninstaller/Manager APK

Time on my hands

Sleep deprivation points

Enough will to live