Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32wl/STM32WL5x_CM0P/RTC/ALRMASSR#0x0
ALRMASSR
Sub seconds value
Mask the most-significant bits starting at this bit
Clear synchronous counter on alarm (Binary mode only)
https://github.com/modm-io/cmsis-svd-stm32