ULN2803 behavior when its output is not connected to any load in this circuit?
It looks as if it's behaving to specification.
A ULN2803 is not a three-state logic buffer as you've drawn it. It's a linear darlington transistor with an open collector. That means it needs about 1.4 V on the base to start turning it on.
Your 5 V VCC2 supply goes through a voltage divider string R1 R2 R3, that without D2 would turn the 2803 on. Its Thevnin equivalent at the input to the 2803 is about 3.3 V from 6.7k.
The 2803 will now turn on, pulling the R3 R1 junction down through D2. Obviously this will decrease the drive to the 2803, and it will start to turn off. It will reach equilibrium when its input is just turning it on enough for the current through D2 to provide enough input voltage to turn it on just enough. Those voltage values look plausible for exactly that.