STM32 gotchas
160. LSI in 'U5 does not work - unless Backup Domain access is enabled before enabling LSI

In most STM32, the low-frequency (and low-precision) internal RC oscillator LSI is in the VDD power domain. As a consequence, if it is e.g. used as RTC clock, RTC stops working when VDD is off, even if RTC itself is powered from VBAT.

However, in the 'U5 family, LSI has been moved to the VBAT (Backup) power domain. This again has a consequence - LSI cannot be enabled until Backup domain access is enabled by setting the PWR_DBPR.DBP bit.

This gotcha has been spotted by user Linas L.