The source of errors is exactly what it says it is, the logic state in that voltage range is not defined the circuit appears to oscillate when the voltage is the unknown range, the Schmidt trigger provides a clean vertical square wave output even if the output from the analog debouncer causes a soft knee when the switch is released.
It may be curious to note that this will also cause the digital circuit to read logic low for a slightly longer period of time than the switch is actually pressed because of the R/C time constant which is what causes the soft knee when the switch is released.