Why does the runt statistics register of Legacy 40/100G Ethernet IP core read back as "X" in simulation?

Due to a change in runt handling behavior in the Intel® Quartus® Prime Software v16.1, the runt statistics register might read back as "X" in simulation in the Legacy 40/100G Ethernet Megacore® function.

Normal core behavior is not affected, but when running the KR4 example testbench, the following messages might be seen:

# DUT 1 Sent                1027 packets
# DUT 2 Received           1027 packets
# Runts in RX data
# DUT 2 Sent                1027 packets
# DUT 1 Received            1027 packets
# Runts in RX data

These messages can be safely ignored.

This problem is fixed starting with the Intel® Quartus® Prime Pro Edition Software version 16.1.1.