I didn't think the Gantt would recalculate the reassignment on the fly like it would in other areas since the assignment is done in the same edit window without having to close it.
On a default task value, since it is fixed duration as soon as you remove someone the hours jump to full as it recalculates against the default calendar at 100% utilization - is this what you are seeing, or what are your task settings (Fixed Units, Duration, or Work?). Also, are these users on different calendars or the same calendar?
Last 2 places to try would be one more place in the gantt - the grid view (slide the divider all the way to the right) and try your re-assignment there, I'm guessing it will still recalculate but try it. Also in the List or Outline view when you do Actions>Edit on a task and reassign it there, does it also recalculate?
Once you let me know about the Fixed setting and Calendars I can try it out as well.