Can you please attach a video demonstrating the issue that is explained? Are you facing it only if you press the button while power up?
Is the issue present even after power up or do the buttons start working after some time?
Can you use BIST (the self test library, requires manual tuning to be enabled) to check the parasitic capacitance of the sensor?
You can then use CapSense_GetSensorCapacitance for measuring the parasitic capacitance of the sensor.
Please make sure that Cp is greater than 5pF and less than 45pF for all sensors.
You can also try manual tuning by setting the parameters obtained from smartsense while the system is working. This way, the sensors will take the best configuration irrespective of the conditions while power up.