Resolved! RM0455: STM32H7A/B Reset value of PWR_SRDCR incorrect
Rev. 5 of RM0455 says "Reset value: 0x0000 4000" for PWR_SRDCR.However, the description "Bits 15:14 VOS[1:0]: 00: scale 3 (default)" contradicts this. And on actual hardware reset value is indeed 0x00002000.