*Sigh* ... I will fix that, but there are lots of positions like that where you can abuse pathing ... Not only with natural positions, but also by blocking with other players, too. This is the greatest drawback of the threat system. The only way I can think of is reducing the bosses pathing to 0.
Maybe you can make an addition to the threat system? If a unit has a new target due to it having a higher threat, start a timer or something and if the unit hasn't started hitting the target after a few seconds, order the unit to set the nearest target as highest threat instead? This would make the "blocked" unit attack the "blocker"? I don't how your threat system actually work of course, it's just a suggestion.