Client MAC Address Absent in Logs

We are using a dedicated vdom for clients that has client reputation enabled, device detection on the interfaces and some additional log options. MACs are logged with the app-control traffic merged into the traffic logs (FGT+FAZ). MACs are not logged from webfilter traffic however. 2 sample consecutive tlog entries from FAZ which show the same traffic via webfilter traffic and app-control traffic: itime=1387758029 date=2013-12-23 time=01:20:29 devid=xxxxxxxxxxxxxxxx vd=xxxxxxxxxxx type=traffic subtype=forward catdesc=" Information Technology" dstcountry=" United States" dstintf=" vlink_xxxxx" dstip=65.54.238.213 dstport=80 duration=91 hostname=" stats1.update.microsoft.com" lanin=2660 lanout=636 level=notice logid=0000000008 osname=" Windows" osversion=" XP (x86)" policyid=2 service=HTTP srccountry=" xxx" srcintf=" VLANxxxx" srcip=xx.xx.xx.100 srcname=xxxxxxxxxx srcport=1474 utmaction=passthrough utmevent=webfilter utmsubtype=ftgd-cat wanin=636 wanoptapptype=http wanout=2660 itime=1387758040 date=2013-12-23 time=01:20:40 devid=xxxxxxxxxxxxxxxx vd=xxxxxxxxxxx type=traffic subtype=forward app=" SOAP" appcat=" Network.Service" appid=16730 applist=" Clients_AF" devtype=" Windows PC" dstcountry=" United States" dstintf=" vlink_xxxxx" dstip=65.54.238.213 dstport=80 duration=101 level=notice logid=0000000013 mastersrcmac=xx:xx:xx:6d:d4:1a osname=" Windows" osversion=" XP (x86)" policyid=2 proto=6 rcvdbyte=844 rcvdpkt=5 sentbyte=2928 sentpkt=5 service=HTTP sessionid=2239257 srccountry=" xxx" srcintf=" VLANxxxx" srcip=xx.xx.xx.100 srcmac=xx:xx:xx:6d:d4:1a srcname=xxxxxxxxxx srcport=1474 status=close trandisp=noop A part of a sample screenshot from FAZ/FGT from this traffic is attached.