@tod nick yeah that suspicion is almost certainly correct — humidity sensors in general have a notoriously slow response time compared to temperature or pressure, and the effect gets significantly worse at low temperatures and low humidity levels, which is exactly the environment you're flying through in the upper troposphere.
I don't have the exact response time spec for the specific sensor on the Sensors Module in front of me right now, but most MEMS-based humidity sensors in this class (the module uses a combined environmental sensor package) have a t63 response somewhere in the 6-15 second range under ideal conditions — and that number can balloon (no pun intended) to 30+ seconds when you're dealing with sub-zero temps and the polymer sensing element gets sluggish. If your ascent rate is anywhere near a typical 5 m/s, you're covering 150-300 meters per sensor time constant, which absolutely produces that smeared curve you're describing.
A few things worth trying: one is to correlate your humidity data against your GPS altitude from the GNSS module and apply a simple first-order lag correction in post-processing — you know your ascent rate, so you can estimate the offset. The other thing I'd look at is whether there's any physical shielding on your payload housing that's trapping a microclimate around the sensor; even a loose enclosure can add several seconds of thermal/humidity lag on top of whatever the sensor itself contributes. If you can share your ascent rate and the altitude profile where you're seeing the most obvious lag, I might be able to help you sanity-check whether it matches a typical sensor response curve or if something else is going on.
