For the First-Word-Fall-Through configurations with Embedded Registers, in the VHDL behavioral model simulation, DOUT powers-up as "x" until the first data falls out. The power-up value of DOUT is correct in the Verilog behavioral model.
This issue is seen only in behavioral simulation. Please use structual simulation model instead of behavioral simulation. The structural model can be generated by selecting Core Generator GUI, under Project Option, Generation Tab.