I am currently using ADuM260N0 for isolating signal coming from FPGA to a Digital output ( like relay , UNL, etc ) , while going through the datasheet of ADuM260N0 , I cannot find the option where VDDI (input supply ) is powered , VDDO ( output supply ) is powered , VIX (input ) is tri state what will be default output of VOX (output ) in N0 & N1 case.
I am attaching the sch below.
In your situation, there is no such a default output, because even if input port is tri-state or floating, there still be a voltage on it although we are not sure how much it is. The logic circuit will figure out the input logic based on this voltage, so does the output logic. The tri-state or floating pin is easy to couple noise, so the output logic is not predictable. A pull up or pull down resistor could be placed on VIX to set the logic you want.