- Plesk is configured with 2 IPs (203.0.113.2 and 203.0.113.3) in Tools & Settings > IP Addresses.
- example.com is configured to use IP 203.0.113.3 in Domains > example.com > Web Hosting Settings.
- A Service Plan Add-on is provisioned via WHMCS by clicking on Orders > <order number> > Products > Change Package. This action changes the IP for example.com from 203.0.113.3 to 203.0.113.2.
This has been reported as bug ID #EXTWHMCS-54, which has been fixed in WHMCS Version 7.8.0 General Availability.
Update WHMCS to version 7.8.0 or as a workaround change the IP to the correct one in Domains > example.com > Web Hosting Settings.