Another comment... If you have a concern with the kinds of small changes in weight we are discussing here, i.e., the kind that come from water content changes in (or on) polymers...
To avoid being confused by the potential for the auto-null to induce changes, track the total weight of the cup rather than zeroing the scale on it. That way, the potential auto-null issue isn't part of the measurement.
In other words, get a stable zero and then weigh the cup and record the weight, don't tare on the cup weight and tack the null changes since the auto-null could be playing with you.
In physics labs, we use $very$ good scales to track things down below a milligram, and while these A&D FX120i scales are great for the money, they are not the equivalent of a laboratory scale used for tracking small differences of big values.
That said, it doesn't mean your observations are not potentially real. Many common polymers do change with humidity.
Hopefully, your polymer cups don't change so fast that they ruin your work. As long as you occasionally watch the tare weight of the cup and the zero, you should be able to keep loading. Pick some interval during long loading sessions and force yourself to watch the tare weight of the cup. As long as you are careful to keep the zero, you should be able to keep your powder charges under control.
Nylons and similar polymers are notorious for absorbing and giving off water. Other polymers don't. It all depends on the chemistry and form of the polymer. Nylon, ABS, acrylic, PET, PBT, polyurethane, and polycarbonate, will all vary enough with humidity for you to notice.
The only polymer with zero water absorption is PTFE, but it can bring a severe static problem. Plastics with very low water absorption are polymers such as PEEK, PPS, PSU, PPSU, PEI, PVDF, PET, PP, and PE. All of these have their pros and cons. YMMV