Flash writes fail after exiting stop3 on stm32u375

[Geoffrey1]

I have an application that uses internal flash for logging.  Unfortunately, after exiting stop3 subsequent writes to flash fail.  Here’s my exit code -- the names of the routines should give a general idea.  I don’t really have a clue what changed and the reference manual is notably sparse on details about the various stop/standby/shutdown modes

 

  SET_BIT(SCB->SCR, ((uint32_t)SCB_SCR_SLEEPDEEP_Msk));

  __DSB();
  __WFI();
  __ISB();

  CLEAR_BIT(SCB->SCR, ((uint32_t)SCB_SCR_SLEEPDEEP_Msk));
  PWR->APCR &= ~PWR_APCR_APC;
  palSetLine(LINE_testpin);

  chSysLock();
  tagPowerRestoreClocksAfterStop3();
  tagPowerPostStop3WakeEventI();
  chSysUnlock();
   // Ensure Flash is awake and not stuck in low-power mode
  FLASH->ACR &= ~(FLASH_ACR_SLEEP_PD); 
  __DSB();