What you're seeing is perhaps due to the output going tri-state. Do you see this behavior on every pin? If not, what is different about what is connected to this pin? Is there a capacitance? Maybe you need a pull-down resistor on the pin.
As a general recommendation: don't leave it floating. You can't expect any level from a floating signal, because it is not defined.
And, because a floating signal is high impedance it can be influenced from any signal nearby.
Maybe you just see coupling effects from another signal.