Source Impedance Consideration for ADC
Here's probably what made it work fine: -

The text says that capacitors must be added and presuming that you have, then the source impedance required by the ADC will become more relaxed. Pretty much most ADCs of this type can tolerate several tens of kohm source impedance if the capacitors fitted are circa 100 nF. There should be details in the MCU data sheet that explain this.
If you haven't used capacitors then you should.
Regards the input leakage current of the MCU, the maximum value is 500 nA and this will create an offset error (due to the 32 kohm source resistance) of 16 mV. If you can live with this (it affects all readings a constant amount) then that's fine. If you find that this DC error is problematic then you might need to buffer the signal with an op-amp. At normal operating temperatures the leakage will be about one-tenth of 500 nA so, is a 1.6 mV DC offset a big deal to you?