Facing problem with Modified domain cookies

BIG-IP ASM cookies are session based and do not get written to disk. When the user is connecting to the application using incognito mode, there is no existing cookie so the violation is not triggered. See K5907: BIG-IP ASM violation: Modified domain cookie 

The most common reason the cookie changes is that the client makes a request to another app on the same domain not passing through the same ASM policy which modifies the cookie. Another common cause for the violation is that the ASM cookie is set with a different expiry than the app's cookie. If you can reproduce the issue, try using your browser's development tools to view the cookies in use for the application and monitor changes to the cookie between ASM sessions.