Will,
Can you do a snmpwalk and verify if the ifPhysAddress is showing the correct MAC Address?
On the newer versions of NPM, snmpwalk is located under Program Files (x86)\SolarWinds\Orion\
Alternatively, you can get it from this KB - http://knowledgebase.solarwinds.com/kb/questions/3145/SolarWinds+SNMP+Walk%3A+A+new+tool+for+collecting+SNMP+MIB+walks
NPM retrieve the information from the SNMP agent. Thus, we have to ensure that the device is returning legitimate information, so that we can isolate where the issue might be with.