The RX statistics logic was wrongly counting the Carrier Extend as the part of the frame.
This is why the 63-byte frame was not counted properly as an undersized frame. A 63-byte frame is a corner case for the stats logic and thus only the 63-byte frames counting was affected. Other undersized frames were counted properly.
To fix this we updated the RX statistics logic to not overlook the Carrier Extend portion.
Please use the attached patch to fix the issue. The patch is for Vivado 2016.1.