In vb 3.8.4 (and probably 3.8.x) there is a missing hook in showthread.php. If you compare showthread.php with forumdisplay.php, you will notice that in forumdisplay.php, the forumdisplay_loggedinuser hook is called twice. This is very convenient for anyone that wants to do processing on the users who are logged in (e.g. change usernames to real names or anything else). My understanding is that the reason the hook is called twice is because there are two sections of code - one for the current user and another for all other logged in users.
showthread.php is missing the hook for the current user. I believe that the following line of code should be placed on line 1772 in showthread.php: