Unless the beer was undercarbonated to start with, it shouldn't absorb the CO2 in the keg. Once it reaches equalibrium, it should stay the same unless there was a leak or there was more CO2 "pushed" into it.
Try putting CO2 pressure on the keg. Untapping it and let it sit for several hours. When you retap it make sure the CO2 tank is shut, but open the valve to let the pressure gage read the keg pressure. If the pressure is lower, then you probably have a leaky keg.
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