After System Reset, if setting RENB=1 before sending packets or toggle RENB with shorter intervals, RDAT is not fully transmitted. Consequently, some of the data might be lost. This issue occurs with function simulation and on the actual device.
This is an issue within the core, and a patch is available to fix this issue. Contact Xilinx Technical Support and open a WebCase to obtain the patch:
Mention this Answer Record 24980 to the Technical Support Engineer.