In the Gigabit Ethernet MAC v5.0 Core, the Tx Byte Valid bit (Bit 20) of the tx_statistics_vector is always stuck at ground.
There is a patch available to fix this issue with the Tx Byte Valid bit of the tx_statistics_vector. To obtain this fix, please install the patch that is available in the LogiCORE Gigabit Ethernet MAC v5.0 Core Release Notes and Known Issues Answer Record (Xilinx Answer 19878).
NOTE: The optional statistics counters do not have this problem. Consequently, this should only be an issue if the Byte Valid bit of the tx_statistics_vector is used to generate your own statistics.