need to understand the difference in delay for rising and falling reference
For the ground to 3.3v swing the voltage across C1 changes from zero to about 0.7 volts where the transistor is turned on.  That takes time.  But for the 3.3v to ground swing, the voltage across C1 drops below 0.7 volts almost immediately on the way down to zero volts.  So the transistor turns off almost immediately.

Redesign the circuit such that the capacitor can charge beyond the Vbe 0.7 limit by adding a resistor in front of the base.  Allow the capacitor to charge just to about 1.4 volts.  That way it should take about as long for the capacitor to discharge from 1.4 to 0.7 volts on its way to zero volts as it does to charge from zero to 0.7 volts on its way to 1.4 volts.