SSH with a Bastion Host - stdio forwarding failed
I didn't have SSH security group configured correctly, when it was changed to accept incoming SSH requests from the Bastion Server's private IP, everything worked as expected.
In my case the Bastion/Jumpbox host was not able to resolve the Hostname (I used hostnames and not IP), so in my case that was a bad DNS entry in my DNS setup, so just make sure the Bastion host can resolve to the same name you are using in your ssh config file.