V1910 can see the MAC of a router on one port but gets no ARP info and no IP addr for that router

I have ten or so V1910 switches distributing internet to all flats in a condo (in stockholm, sweden).
Each tenant is obliged to have a router, an always-on NAT router, as the only device connected to the condominium network. The condo network is connected to a common internet provider through a zyxel usg2000.
DHCP is done by a Zyxel USG 2000 feeding a top switch, so all private routers get an address in 10.0.0.0/16 from the USG2000.
For most tenants, Network->ARP and select by port shows the ethernet address of their respecive routers, and Network->ARP Management and select by port shows the IP address for their routers. Except for one tenant, where I can only see a MAC address (verified to be correct, and verified by disconnect/refresh/reconnect/refresh to be actually seen live) in the MAC list, but no bound IP address in ARP management.
The tenant is cantankerous to put it mildly, so I'm searching for clever ideas to avoid having the tenant try and try again for testing.
His router is a Netgear WND3700.
1.. Can I trust the V1910 web management gui to give me the correct info?
2. What could be the problem that leads to ARP management not to work for his connection??