I wonder if the DHCP servers in WAN and LAN are the same, this could create some confusion in the lease because the IP believes he has to hols the lease for a given time.
We need more background information about this network:
Are the LAN and WAN networks assigned an address in the same subnet?
What type of VPN are you using to connect the two sites? AND how much bandwidth does it have?
Where are the DHCP servers located?
What type of security do you have in place between the sites?
How are you routing traffic between the sites?