Was it a 6.x.x.x address? "When in Passthrough or Routed/NAT mode in Single LAN the MX will source traffic from a 6.X.X.X address for services such as Syslog, Netflow, RADIUS access requests and potentially others." https://documentation.meraki.com/MX/Networks_and_Routing/Passthrough_Mode_on_the_MX_Security_Appliance_and_Z-series_Teleworker_Gateway#Considerations_for_VPN_and_Other_Features
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