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.

On the macOS end - you have unix heritage and tools, so let's look at how you pick apart which network interfaces have grabbed which addresses and match what the system was told in response to DHCP broadcast. You could enable packet capture / Wireshark if needed, but here are the quick tools I'd use:

ifconfig -uv
networksetup -listallhardwareports

Pay attention to the en0 / en5 / en whatever since those are ethernet and not tunnels or wireless discovery interfaces.

for me I have ethernet USB adapter (en4) and WiFi (en0)

ipconfig getpacket en0

So the likely values you want from the longer list are:

mac:~ me$ ipconfig getpacket en0 | grep -e addr -e server -e xid
xid = 0xc9d9ceb7
ciaddr = 192.168.1.27
yiaddr = 192.168.1.27
siaddr = 0.0.0.0
giaddr = 0.0.0.0
chaddr = a8:66:7f:4:d:1
server_identifier (ip): 192.168.1.1
domain_name_server (ip_mult): {192.168.1.1, 1.1.1.1}

At this point you'll know if another device is answering DHCP broadcasts or if the Mac has gotten an IP address it wasn't assigned and messing up your network thoroughly.