AWS SSM SSH with host alias not working as expected

Question

I'm wondering why this command works (1):

ssh -i .ssh/my-dev.pem ubuntu@i-1234abc

But this command fails with the error below (2):

ssh myec2

The error is:

An error occurred (TargetNotConnected) when calling the StartSession operation: myec2 is not connected.
Connection closed by UNKNOWN port 65535

This is my .ssh/config:

Host myec2
    User ubuntu
    HostName i-1234abc
    IdentityFile ~/.ssh/my-dev.pem
    ProxyCommand sh -c "aws --profile myawsprofile --region us-east-1 ssm start-session --target %h --document-name AWS-StartSSHSession --parameters 'portNumber=%p'"
Host i-* mi-*
    IdentityFile ~/.ssh/my-dev.pem
    ProxyCommand sh -c "aws --profile myawsprofile --region us-east-1 ssm start-session --target %h --document-name AWS-StartSSHSession --parameters 'portNumber=%p'"

Running command (2) with the above config, should be the same as command (1), by my understanding. So there's something I don't understand.