Plus Address fails for aliases?

This seems like it should be a given but all I get is:

Remote Server returned: 'The recipient does not exist'

Conditions:

Works: username+Vendors@mydomain.com

Fails: useralias+Vendors@mydomain.com

Where useralias is under the alias tab and sends only to username@mydomain.com

It seems this should be have equivalent behavior. Looking on the docs for plus addressing, it doesn't say "will not work for email aliases".

Clarification?

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