That is typical. They are not exactly “moisture” packets though. They are designed to remove oxygen which helps keep bacteria and mold from growing. The packets work by consuming the oxygen, and as they do, a chemical reaction takes place inside the oxygen absorber packet, thus resulting in heat. So, you know they are still good if they are producing heat when opened to air. That is also why you only want to open the package for a brief period and re-vacuum seal any packets that are not going to be used in a 30 minute or less timeframe, to keep them from depleting too much of their use as it does have a finite amount of use.
For another example, oxygen absorber packets work kind of like “hot hands” hand warmers that you put in your pockets to keep your hands warm. They both consume oxygen and produce heat, obviously for 2 different results, but ultimately, kind of a similar process.
Hopefully that helps explain things a bit! Let us know if you have any other questions!