Using 100k+220k resistors in the voltage divider, the AD converter input is 2.9V at a maximum battery voltage of 4.2V.
Since the full scale of the AD converter is 2.5V, the input must be less than 2.5V.
If you show me your sketch, I may be able to give you some other advice.
edit
I think chatgpt is assuming that VDD (3.3V) is used as the reference voltage for AD conversion. if VDD is used, voltage below 3.3V cannot be measured.