Here's my guess as to what's happened:
Connecting to Cosmos DB Gremlin endpoint requires a host name which includes the Cosmos DB account name.
Cosmos DB has a single public IP address per region where inbound connections are load balanced across gateways instances. The gateway instance will attempt resolves the account by parsing it from the host name of the inbound connection.
This commit changed the way host URIs were built, switching from using host name to using the address instead.
So without the full host name, the gateway can't resolve the account name, resulting in NotFound error.
Assuming this is what's happening (will need to confirm), it seems like the change to using IP addresses should be behind a configuration?
Oliver