This does depend on how your PFD became inflated. If you do end up in the water, and your device is inflated via the CO2 cylinder, over time the CO2 gas will escape the bladder; this is totally normal and should be expected. It will require topping up of the bladder via the oral inflation tube; this will not be immediate and is likely to be necessary after a few hours.
If your inflatable was inflated via the oral tube with regular air (from your lungs, or an air compressor), you should expect to see the bladder maintain its firmness for a much longer period of time (we recommend a minimum of 16 hours when bladders are checked for air retention). If you are in changing conditions, particularly a large temperature drop, this can also affect the firmness of the bladder; in all cases we (Mustang Survival) recommend topping off the bladder via the oral inflation tube.
The science behind CO2 filled bladders losing pressure is due to the nature of CO2. CO2 can pass through materials easier than air, which can lead to a reduction in pressure after a few hours.
A bladder become less firm with temperature drops due to the nature of a gas (such as air, CO2, oxygen); when a gas gets colder, it reduces its volume. This means that a bladder that is firm at 20C (68F) will feel much softer if it is cooled to 10C (50F), and much firmer if warmed to 30C (86F); no gas is added or lost, but the volume of the gas does change. The ambient temperature will affect the firmness of a bladder, even if there are no leaks. Check your product's user manual for specifics on recommended operating temperatures