Increase the sample size from fifty & the averages of your parameter estimates over many simulations should get closer still to the true ones—that's what *asymptotically* unbiased means.

[As @Joe's pointed out, bias (at least "bias" without further qualification—there is such a thing as median-unbiasedness) is the expected difference between the true value & its estimate, so you should be looking at the means rather than the medians in your simulations, if it's this you want to assess.]