This was once a bug that got fixed. In those cases, the message was appearing because the keys were "lost" before connecting to the actual node. In your case, the system works as intended and it warns you that you are connecting to another machine. In case of an IP conflict, you how much bigger problems than SSH though.
You either have to ensure your static IPs are not part of your DHCP pool that is assigned at all or set PVE host(s) with DHCP as well.