Leontes is already a friend of mine in-game (Fantasy Tags) but not a friend on my Steam list.
If I go via my normal friends list and click his name there are 4 options including “Steam profile”, which if I press it, goes to his Steam profile correctly.
If however I search to ‘add friend’ “leontes” first, it brings him back as an in-game results screen where he is highlighted in gold and it says “Already friend”. If I then click his name, I only get 2 options “Avoid for Online” and “Steam profile”, however pressing “Steam profile” here just gives the acceptance bleep sound effect, but does not bring up his profile correctly.