, I already had this problem... The only solution I found by myself is:
1) Backup all your data from the internal memory (i.e. using FTP or whatever method you like the most)
2) Download latest MsmDownloadTool for your device, latest driver for Qualcomm EDL and the latest OTA from the OnePlus site ( we will need it later)
3) Power off your device before connecting to the PC. Hold VOL+ and VOL- and connect it to your PC, you should hear the sound of a connected device and the phone's screen is black.
4) Check if the device is properly connected in device manager. (If it doesn't work select manually the driver)
5) Start MsmDownloadTool V4.0.exe and if it asks the login choose from User Type "Others"
6) Click on the enum button a bunch of times and if your PC is saying "Connected" you can start the downgrade... Be wary that this procedure wipes all the data inside and locks again the bootloader!!
7) When it reboots it's back to OOS 11. Skip all configuration steps (it doesn't matter, we have to unlock the bootloader)
8) Proceed to unlock the bootloader and installing TWRP as the OP said
9) Transfer the OTA from your PC to TWRP and install it but make sure to tick the "Keep TWRP after rom install". Go back to the main menu and select reboot select the correct slot in which you just installed the OTA and select system.
10) When the phone boots up, it will be on OOS12 but with TWRP.
11) Reboot in recovery once more and flash again the OTA with the same option as before. (It's not required, but if you want a bootable phone if you mess around with the system, is a good thing to have)
If you want you can flash magisk 25.2 as normally in recovery too...