After making a change inside of the WHMCS admin area, your session will expire and ask you to login again with the full username and password.
If WHMCS is automatically logging you out of the admin area interface after making changes it is likely that your IP is changing. By default, WHMCS has a session IP check that is used to protect against cookie/session hijacking. It is suggested that you do not disable this session IP check, however, if you are using a special proxy (like CloudFlare for example) you will need to add these IP's to the WHMCS settings as Trusted Proxy IP's.
There are two workarounds to stop WHMCS from automatically signing out after making changes:
1.) Not Recommended - You can sign into WHMCS and go to System Settings >> General Settings >> Security, and then click on the box for: Disable Session IP Check
2.) Determine the IP's that you are using to access your domain, and add those IP's as Trusted Proxy IP's. More information on this can be found here: https://docs.whmcs.com/Trusted_Proxy_Settings
If you are using CloudFlare, you will need to add the CloudFlare IP's to this list. The CloudFlare IP list can be found here: https://www.cloudflare.com/ips/