This is not unusual when analysing certain types of food samples and we have also experienced this type of response in our laboratory.
The beta-galactosidase will rapidly hydrolyse lactose but also displays small amounts of activity on galacto-oligosaccharides which will contribute to this creep.
The MegaCalc contains a creep calculator for this reason.
If there is a “creep reaction” after the initial rapid reaction (A3; absorbance read at 15 minutes after the addition of beta-galactosidase), then measure additional absorbance values at 20 and 25 min. Enter these values into the “creep reaction” calculation. The program will automatically extrapolate to time zero and calculate the correct concentration.