Here's the problem - at the point when the output changes state and the DIS pin releases (goes to high impedance), the direction of electron flow into the cap does not change. The DIS pin has pulled the cap top end to GND, and when it releases, R1 also is connected to GND. The other end of the cap is at Vcc, so it isn't changing. Thus, the cap keeps charging up to a potential of Vcc volts, but at a slower rate since now it is charging through both R1 and R2 in series.
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