I might be wrong but I belive LowPowerTicker was originally something unique to the SiLabs targets?
Perhaps OS6.2 has included it now, but maybe not for all targets?
The F103 needs a lot of GPIO setting (that’s to AnalogIn) to achieve real low power functions.
There’s more info here;
The moment you assign a GPIO to a function, use deep sleep mode (via Mbed OS or manually) then there will be a power trade off.
You may need to go more ‘Bare Metal’ to get really good low power.
I use the 103 in Bare Metal to achieve really low power projects where I need sub 10uA deepsleep.