I managed to fix the annoying fingerprint issue (the one reported above by me, and by other users on the first few pages of this thread). The fingerprint sensor was acting like an "enter key" whenever I touched it, which was quite annyoing).
It was fixed after completely restarting the whole process, by reflashing the latest MIUI rom (12.5.3 Global) and then repeating all the steps to install ArrowOS. Here's a list of steps, if you're having the same issue or any other issues that might pop up for just some users:
1 - Use MiFlash20210226 to flash the ROM downloaded from xiaomifirmwareupdater website:
"MIUI V12.5.3.0.RJGMIXM Stable Official Update for POCO X3 NFC Global (surya)"
The archive name is: surya_global_images_V12.5.3.0.RJGMIXM_20211013.0000.00_11.0_global_9eca9fae68.tgz
Size: 5.2 GB (it's the fastboot type, not the recovery type).
Note 1: if you don't know how to do this, look for "[GUIDE]⚙ USE XIAOMI FLASH TOOL" here on XDA, by user
@NOSS8
Note 2: It's normal to receive an error at the end when using the "clean all" option, right when the smartphone automatically reboots while flashing. As long as it's taking a while to flash all the necessary data, it should be fine.
2 - Don't know if this is necessary, but I booted into the MIUI 12.5.3 system, skipped most configs, then I let it finish the initial configuration. Made sure it's the last update avaliable through OTA.
3 - Reboot into fastboot (hold power +volume down).
4 - I used the "Minimal ADB and Fastboot" tool in order to use the fastboot command to flash the latest TWRP compatible with Android 12 by
@brigudav
Currently, the file is called: twrp-3.5.2_10-12-surya-a12.img and you can find it here on XDA.
Note: Boot into TWRP right away after flashing it, don't let your smartphone boot into the MIUI or it'll be overwritten.
5 - While in TWRP, go to WIPE > Advanced wipe > Select "Dalvik / ART cache" + "Cache" + "Data" and confirm the action.
Press the home button to return to TWRP main menu.
6 - While in TWRP, use "install" to flash the zip archive of the latest firmware package for MIUI V12.5.3.0 (Global).
Currently, it's: fw_surya_miui_SURYAGlobal_V12.5.3.0.RJGMIXM_624ada8a33_11.0.zip also available through the xiaomifirmwareupdater website.
Press the home button to return to TWRP main menu.
Note: you can use your PC to send these zips into your smartphone's "Download" folder while in TWRP.
7 - Also while in TWRP, use "install" to flash the zip archive of the latest official ArrowOS (Gapps version) with android 12.
Note: Don't forget to go into WIPE > "format data", and typing "yes" to confirm, after flashing the ArrowOS Zip archive.
8 - Reboot system and you're done.