The True client IP feature in app the profile only accepts IP address while it should be accepting custom headers.
This is a UI bug which will be addressed in 22.1.7-2p2 and later maintenance releases.
The workaround is to configure this feature via the CLI
[admin:<controller-IP>]: > configure applicationprofile <app-profile-name>
[admin:<controller-IP>]: applicationprofile:http_profile:true_client_ip> headers <custom-header>
[admin:<controller-IP>]: applicationprofile:http_profile:true_client_ip> save
[admin:<controller-IP>]: applicationprofile:http_profile> save
[admin:<controller-IP>]: applicationprofile> save