When a player follows a friend to a game through the homepage or party chat follow button, it will not update the player FollowUserId in-game property correctly and so we can’t detect who the player followed.
How to reproduce
To reproduce this bug, you will need a place with a script that prints the FollowUserId of a player that joins the game. The quick follow through the homepage and the party chat follow will not print the FollowUserID of a player that joins, while the option from a players profile does correctly print this.
Join options that will not print the FollowUserId: