Need help with a Redmi 12 5G that appears to be stuck in a locked-bootloader Fastboot state.

What happened:

• Flashed official firmware (matching device variant as far as I know).

• After flashing, the bootloader became locked.

• Phone briefly shows the Redmi logo on power-on, then immediately goes to Fastboot.

• Hardware key combinations are not getting me into recovery.

• An authorized Xiaomi service center was unable to fix it and returned the device.

Fastboot information:

• fastboot devices detects the phone.

• fastboot getvar product returns river.

• fastboot getvar unlocked returns no.

• fastboot oem device-info shows:

  • Device unlocked: false

  • Device critical unlocked: false

  • Verity mode: true

Important output from fastboot getvar all:

• current-slot:a

• slot-unbootable:a:yes

• slot-successful:a:no

• slot-unbootable:b:no

• slot-successful:b:no

• secure:yes

When I try:

fastboot set_active b

I get:

FAILED (remote: 'Slot Change is not allowed in Lock State')

Questions:

1. Has anyone recovered a Xiaomi/Redmi device from this exact state?

2. Is there any legitimate way to restore slot metadata or recover the device when the bootloader is locked and Fastboot is the only accessible mode?

3. Does this point more toward corrupted boot metadata/firmware, or could it still indicate a hardware issue?

4. If you've seen this before, what was the successful fix?

Any insight from people experienced with Xiaomi A/B devices, EDL recovery, or bootloader issues would be appreciated.