Substitutions not working in Connections language override

I am trying to change the text string for

CONNECTIONS_YOUR_CONNECTION_REQUEST_WITH_USER_IS_PENDING

As it is now, the string is:

Your connection request with [user] is pending acceptance

I want to replace [user] with [username], but when I do that, the rendered result is the text " [username]". The only substitution that seems to work for this language string is [user]. How can I get username to show up here instead of that little avatar picture of the user?

It's not passed through substitutions. [user] is just a replacement specifically for that string to insert the avatar. You won't be able to supply anything other than [user].