How can I figure out why a MacBook Pro running Mojave causes network issues with our corporate Wi-Fi network? We have a been problems with new Macbook Pro and Cisco Wireless. We able to see that some time the Macbook assuming the IP Address of network default gateway. 

Looking for mac address of the default gateway, we see the Macbook MAC Address, not the MAC Address from network default gateway.

This problem occurs and is solved without any configuration. 

Has anyone seen similar issues or know how to resolve it?
Make sure your DHCP pool does not overlap with your default gateway.  Thats the most common problem with what you are describing.