custom instruction not working in 13.1.3

I think this is a case where the documentation isn't explicit that you can't do it, but your use of 'b' is not what they were expecting.

Taking your code as-is, both Qsys and Quartus have warnings about 'b' (for example, "Explicitly unconnected" in the Quartus Port Connectivity Checks report). I'm using 14.0 but if you read the top-level Qsys file, you'll see the port not connected.

If you add the 'readrb', the warnings go away and the port is connected in the top-level.

Not sure if it will behave as you expect however.