Why does my STM32F427VG not allow erasing the top half of flash?

[MKenn.1]

STM32F427VGT6. Should be 1MB flash, right?

Then what's up with this?

• Mass erase: works, erases sectors 0-11
• Write: works, can write to full 1MB
• Single sector erase: doesn't work, either stlink or from firmware running on the device (see attached screenshot of STM32CubeProgrammer).

What's going on with this MCU? Other identical MCUs on identical boards work just fine.

[Tesla DeLorean]

The FLASH size tested is burned into OTP at the factory at test time.

Mouser's decoder wheel says 1MB

STM32F427VGT6TR

I think the VI part has 2MB

STM32F427VIT6TR

[MKenn.1]

Yes, I'm aware it should have 1MB of flash. I'm failing to erase the upper 512K of that 1MB.