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Manfred is correct. The high voltage input has a voltage divider at its input, scaling the high voltage down to a voltage appropriate for the ADC. A side effect of this is that when nothing is connected at the input, the ADC input is clamped to zero.
On the auxiliary input, there is no input voltage divider, the path from the terminal to the ADC has only a few protection components attached. Part of the protection is a set of diodes connected to the power rails, which will leak very small amounts of charge. I believe it’s this leakage that’s causing the drift you are seeing.
Anyway, long story short, it’s normal :)