Yes, but it’s really about the first opamp in the analog front end, not undersampling. We have an excellent analog anti-aliasing filter in the JS220 to prevent undersampling issues and aliasing. However, if the first-stage opamp cannot keep up due to bandwidth or slew rate, then it could distort the signal. This is still a hypothesis that needs more testing.
One way to test this out is to add an additional 10 µA or more of effective capacitance at the target.
Remember to derate capacitance for the working voltage. This reduces system bandwidth to align with the JS220’s measurement bandwidth. If my hypothesis is correct, then the 18 mA range measurement with this additional target capacitance should be closer to existing 46 nC and 48 nC measurements from the 180 µA and 18 µA ranges.