How to stop/reset the watchdog timer from low-power sleep mode on an STM32L0C8?
The watchdog is only stopped on system reset. This way you can't accidentally turn it off. 

If you want to use sleep modes and watchdog, you will have to set the watchdog the slowest clock, and wake-up regularly to restart the watchdog.

This is valid for almost all microcontrollers on the market that have an independently clocked watchdog.