System Generator: How to configure the CORDIC divider block?
From the latency on the cordic block, that looks as if you've chosen to use a single processing element - that will produce results of limited accuracy.  Very limited it appears :)

I tried using 10 elements for example, with [zeros (1,9) 1] for the "latency per processing element" - to get back to your 21 tick latency.  Making sure I ran the simulation long enough (oops), I then got a result (using UFixed_16_11 inputs) of 0.501953125.  Which is closer...

Does that help?