When an input is left unconnected, it's called "floating". Unless there's an internal pull-up or pull-down or has specific behavior defined in the datasheet, its state is indeterminate. It might be high, it might be low, it might switch unexpectedly. The ambient temperature or humidity might even be a factor.
For your IC, the datasheet defines a LOW input as a max of 0.8V and a high input as a min of 2V. What voltage is an unconnected input?