When memberlist is being sorted by user last visit, there're users pop upon pages where they shouldn't be. The problem is the users list building logic.

1st, it selects a set of users from the database counted by topics_per_page value sorted by user_last_visit column.

2nd, it selects corresponding data from sessions database table.

3rd, it assigns $row['last_visit'] to $row['session_time'] if exists, otherwise to $row['user_last_active'].

The last step is the cause as the value of $row['session_time'] can be either greater or lower than $row['user_last_active'].

So it displays incorrect result on members list pages where (f.e. sorting in ascending order) there're users on page N have rendered last visit time greater than users on page N+1 and vise versa.