A shooting buddy forwarded an image of a 3D printed powder jug cap that was set up for those hygrometers. My first thought was the same as Rich's. If enough people put cheap electrical devices in powder jugs, eventually there will be a problem.
The Humidity packs work. I still dump powder into a 3 gallon paint bucket that's been hit with used dryer sheets and mix it before use.
Where I store my powder, the humidity drops from 55% to 42-44% over the course of a couple years. I've started using a couple large desiccant packs in the sealed 3 gallon paint bucket to age the powder down to 44% before I use it. It speeds the powder up noticeably but the moisture level stays stable. After mixing before use, I still check the current powder moisture with the Kestrel.
RH has become the popular designator for powder moisture but it's problematic. It changes significantly with small changes in temperature.
Most don't/didn't work in an industry where the interaction of air and water were important and won't have something like this on their desktop. Point 1 was set up as a reference with a temperature of 60 degrees and 50% relative humidity. Point 2 is the same amount of moisture but at 50 degrees. The relative humidity has changed to 72%. This is because the amount of water it takes to saturate air rises significantly with temperature. Notice that the dew point didn't change. It makes a much better indication of the amount of moisture in air.
Dew point isn't perfect either, just better. Below, I changed the air pressure instead of the temperature. The pressure change is about 2000' of elevation. The relative humidity doesn't change as much as with the 10 degree temperature change and the dew point also changes a small amount. The local pressure doesn't change this much, but when comparing your numbers with someone else's, it'd be a good idea to check this.
The performance of the equipment I worked on for 40 years changed strongly with air temperature and humidity. The electronic humidity monitoring instruments were easily the least reliable instruments. Those were industrial grade instruments and we had calibration standards. The Kestrel Drop is a consumer grade toy compared to them, and the Amazon instruments are several steps below that. I'm not writing that to dump on Kestrel. I think the Kestrel Drop is a useful tool for fringe hand loaders. I'm just trying to convey an appropriate sense of skepticism.
Another big step in the chain is the instruments measure the moisture level in the air above the powder, not the moisture of the powder. This has a couple consequences. The first is the air moisture level takes time on the order of several hours to stabilize after opening a powder jug or dumping it in a paint bucket and sealing it. The next is what happens in the air is fast compared to what's going on in the powder. So, even with mixing, changes in the actual powder moisture takes days. Without mixing, there will be layers of lower or higher moisture. Next, while there is a relationship between the amount of moisture in the powder and the air above it, they are not the same and that relationship likely varies with temperature.
The short version of all that is trend towards being suspicious of the instruments, start moisture measurements the day before, and don't chase changes of a few percent. The use of dew point rather than RH will also likely save some frustration.
Dew point is also another bit of reality. It's measured by lowering the temperature on a mirror until it clouds with condensed water. Below the dew point, the same thing is happening in a cartridge or jug. The amount of water in the powder is much greater than the air. I'm guessing this is one of the mechanisms that causes powder clumps. I've been meaning to test if there is a kink in the powder velocity/temperature sensitivity at the dew point but it hasn't made it to the top of the list yet. In the meantime, I store my powder in conditions above it's likely dew point. With fresh powder, that's around 50 degrees.
