Problem with SV Interfaces in Quartus

Possibly review the "Register Power-Up Values in Altera Devices" section of the recommended coding guidelines: http://www.altera.com/literature/hb/qts/qts_qii51007.pdf

If your real code has clock and resets, add them in to try to get your bearings. On the surface, what you're trying to do isn't that different from Example 13-37.

For example, regarding your 'data_creator' module above, I believe you will get good results if instead of the 'initial' statement you had it as the reset condition.

....
if( ~reset_n ) 
 test_interface.some_byte = 8'b1010_1010;
else 
 test_interface.some_byte = foo;
....

Regarding using interfaces to clean up your code: I believe that your 'initial' statements (if you ever get them working) can be placed in the interface definition itself instead of within the instantiating module. Then, you can also add task methods for manipulating the ports.