Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32wl/STM32WL5x_CM4/TZSC/TZSC_MPCWM3_UPWMR#0x0
Unprivileged Water Mark 3 register
LGTH
https://github.com/modm-io/cmsis-svd-stm32