Resolved! Backup domain is not reset by setting RCC_BDCR.BDRST, unless PWR_CR.DBP is set, too
In an STM32F427, according to the Backup domain reset subchapter of RCC chapter of RM0090rev.19, backup domain can be reset by setting RCC_BDCR.BDRST. There is no other condition to this mentioned in that subchapter nor elsewhere in related material ...