[OnePlus 12] jhonyrod:
Had exactly the same issue, you're going to need a backup of

YOUR

original persist partition and restore it using your preferred way, though it may involve some back and forth between your current state of such partition and your backup. In my case I also did some swapping of the fp sensor calibration data between both versions of my backups (original and damaged) by backing it up and transferring it with MiXplorer with root access.

The calibration data file can be found in /mnt/vendor/persist/fingerprint/fingerprint_cali.bin when using a root explorer/terminal on-device or probably in just /fingerprint/fingerprint_cali.bin when mounting/browsing the backup image or the persist partition directly. I will attach mine but I'm sure your mileage will vary, use with caution and only as a last resort step as I'll describe in more detail farther down in this post.

It is strongly discouraged to use random persist images from other phones; as you can probably tell, you're missing the calibration data for your specific fp sensor and hardware configuration/combination, i.e. your combination of sensor/display and, as far as I reckon, even stuff like screen protectors and/or any subtle imperfections in build quality of the aforementioned chain. These might affect how your fp is read by the optical fp sensor, though I might be exaggerating how much it actually affects the calibration point.

Even after that, flashing other phones' persist parition might come with possible issues with WiFi, Bluetooth, and maybe even IMEI information.

Don't quote me on any of what I've said about the dangers of replacing your persist partition, though it is always best to exercise ample caution when doing these kind of operations as they have inherent risks.

Also, obligatory reminder and disclaimer: I am and will

not

be responsible for any current or further damages you might inflict on your device when performing any of the operations described in this post. This is based on personal experience, previous research and some informed guesses, and I'm sharing these in good faith and in hopes that you, and any other person experiencing this issue, might restore your device's lost functionality.

After all of this is said, I think your best course of action is the following:

If you have a backup of your original persist partition:

Well, you can pretty much try any combination of the steps described above, even replacing the calibration data file or the whole persist partition with some other phone's. As long as you keep an original, unmodified version of your persist partition's backup, and you don't mess with any other device-specific partitions, you can always restore your backup. It might also come in handy to backup your current damaged persist partition if you want to avoid re-flashing your device's firmware repeatedly.

If you don't have a backup of your original persist partition, then I'd try the following in increasing degrees of risk:

Get your hands on the calibration tool:

Visit a repair shop or even an official OnePlus service center:

If you're lucky they'll have access to the (as far as I can tell) very inexpensive calibration tool and you most likely won't have to pay much to have them run the calibration routine.

Buy the tool yourself:

AliExpress had a ton listed for OnePlus and other BBK devices for fairly cheap (<$30 USD), but you'll need to do your own research as to which one is the right one for our OnePlus 12, as there are a couple of brands/variants available and I didn't investigate to the point where I could've determined which one is the correct one.

I don't think you can damage your device in any way by trying this, as the calibration is only done in software and it will most likely be rejected if you're using the wrong tool. Even then, if it's not rejected and your fp sensor is still not working after several (and I mean several, as shown in some YouTube clips of the process) calibration attempts, you can always try a different calibration tool.

Replace the calibration data with the one attached below or any other:

This will most likely not result in any loss of data as your calibration data is already damaged. You can backup your current calibration data file (if any) if you want to be cautious, but, obviously, it's not currently working, so if you restore it you'll have a fp sensor with the same functionality as you have now: non-working.

Replace the entire persist partition:

As a last resort you can try to flash a different phone's persist partition, but not before backing up your current one, even if it's missing the fp sensor calibration data, it still contains other important information about your device. I don't feel comfortable either with anyone trying this or with sharing my own persist partition image, so you'll have to look one up yourself. Proceed with caution.

TL;DR: Don't be lazy, read the whole post, it's sensitive stuff what we're dealing with here. Either read carefully or don't attempt anything, you've been warned.

If you have further questions on how to perform anything in this guide I'll do my best to help out.

PS: I borked the list while editing and can't seem to be able to fix it, any moderator that could help me out will be appreciated. Points 2 and 4 are not separate points but continuations of the previous point's paragraph.