Here are a few things to look for.
If you look at the user's details in the applsys.fnd_user_preferences table:
WHERE user_name = :user_name
AND preference_name = 'NEW_WINDOW_FLAG';
The PREFERENCE_VALUE was set to 'R' for the user.
To permanently change the value, remove the tick, and go to the Top Hat in the toolbar to swap resps. Choose a different responsibility.
The checkbox is only updateable when no other forms are open.
The function "Navigator: Disable Multiform" causes the option to be greyed out. You can add a function exclusion to the responsibility, or if it's a custom menu then remove it completely.