It looks like there is a limitation with this connector, due to SFTP Connector is hosted on the Power Automate service, which is a multi-tenant service. Even when integrated with a Virtual Network (VNet) and a NAT gateway, the connector still communicates with the SFTP server over the internet, hence it uses the Power Automate public IP.
- https://techcommunity.microsoft.com/t5/azure-integration-services-blog/static-ip-of-logic-app-standard-using-nat-gateway/ba-p/3721443
- https://learn.microsoft.com/en-us/connectors/sftpwithssh/
Other Alternative It's if you want to whitelist this workflow, you can add the IP public of power automate to your service:
https://learn.microsoft.com/en-us/power-automate/ip-address-configuration