Forticlient adding a route to the end device

There are few clarifications required to start to answer your question.
Typically, if you are using 10.0.0.0/16 internally you would introduce NAT at your edge to protect your self from routing issues, as the one you describe.

Why do you have to use the same 10.0.0.0/16 for Guest WiFi network at the first place? It's for guests so wouldn't matter what IP range they get, would it? You're supposed to separate guest networks from corp/internal network for security reasons. Assigning a different subnet like 10.255.0.0/16 makes traffic/security management and troubleshooting much easier and avoid headaches.

So basically this issue happened when a user of ours that is a road warrior and he went to a company that used the 10.0.0.0/16 has guest wifi.

That is my issue.

Some devices will cache the last know IP.
In most cases this is a function of the Operating System.
For Windows you can test this by running a ipconfig to see the current value,  ipconfig /release to force a stack reset, followed by ipconfig /renew, to force a DHCP discovery.