Air loss is not impossible, but very unlikely. Usually this is the result of the effect of the cooling water (tempering). If the difference between air and water temperature is significant, the warm air used to fill the boat cools and contracts on contact with cold water. As a result the air pressure inside the boat drops and it feels less firm. In this case, you should simply inflate the packraft again a little after the first half hour on the water or temper it already before starting to paddle.
If you are still unsure if your boat is losing air, a leak test can be conducted. First, fully inflate the boat and ensure all valves are tightly secured. Then, leave it overnight in a temperature-controlled room. The following morning, it should still be fully inflated. If not, there may be a leak that needs to be identified and repaired.