This rather surprising "feature" is best "explained"by quoting from ST's own presentation:

New GPIOx_ASCR control register has been introduced on STM32L47x/48x devices to control analog interconnection between an I/O pin and ADC input. This register has to be configured before the ADC conversions are started to correctly bring the signal to the ADC input. This register is removed on other Categories of STM32L4 devices and the analog interconnection will be enabled automatically when an ADC channel is selected.

Don't ask me, why.