[OnePlus Nord N20 5G] beatbreakee:
Hey, i noticed ONE step you were lacking in your steps taken to unlock your bootloader. Did you remember to flash the DM-Verity disabler? For most android 12 devices, it has become CRITICAL that you disable this, if you unlock your bootloader AND install a custom recovery! You dont have to do it, if just rooting, but if you installed TWRP (or other non stock recovery), with dm-verity still enabled, you will basically brick your device. DM-verity uses its RSA key hash to verify ANY partitions blocks AS THEY ARE ACCESSED! Meaning it does its security checks in real time, and if it sees a single block has been changed, it will get a different hash response and will stop your progress dead in its tracks! Sometimes without warning, and if your device was in the process of attempting a GSI/GSU flash, then the recovery partition would fail DM-Verity's checks, which when a partitions data fails to match EXACTLY to the hash/signature that it is expecting, it is already loading as a low level flasher, and does not have any protection thru Fastboot or EDL, to roll the "currently being installed" CHANGES TO THE FILE SYSTEM, back! It just crashes itself leaving your device with partitions mismatching, data corrupted, and chaos ensuing cuz you might have a new kernel, custom recovery, and old stock image all simultaneously existing on your main partition. You see how that would be a nightmare? And with Oneplus starting to push security updates without permission, you will slowly see more and more Fastboot functionality removed, until its like my 10T 5G - that has NO fastboot at all other than "FASTBOOTd" which is actually just a glorified recovery. No flashing capability, unless specific permissions are enabled in the HAL config... (which cant be done without unlocked bootloader, root, and a bootable device!)

But make no mistake... they are coming for your bootloader! If you havent unlocked and you are considering, i would advise EVERYONE to flip that dev option toggle, and unlock while you can! Cuz once they remove your access to fastboot, you are at the mercy of hoping an exploit becomes available to circumvent system checks, and let you downgrade. Although most downgrades will STILL keep the radio fw and others used for informing the OS that you have updated those components. Heres more about DM-Verity, but make note, that EVERY Android 12 device, (android 11 and 10 as well) has DM-Verity ENABLED from the factory. So all you bootloader unlocking, custom recovery flashing, ROM flashing ladies and gents out there, ALWAYS REMEMBER to read the details, and responses from ppl who might be having problems, cuz not one problem with these devices is UNIQUE! Some ppl may just not have attempted whatever triggers the error. But 'DM-VERITY' will brick your device, mid flash if it doesnt get the response it was expecting, to verify that your partitions arent corrupt or modified!

To me, THIS would almost definitely apply to your situation, because ESPECIALLY Samsung devices, but others as well, have DM-verity, and VB_meta active, and those are parts of the Trust Zone's secure boot library! And if they dont like what you're doing, they will MAKE SURE that you are stopped, and also make sure that you can never do it again! ( or at least make it damn hard to try again, such as what it sounds like is happening to you!

Hope some of that info helped you discover what may be the problem.

DM-Verity