I have noted that when I have a long list (say >20 ) of tasks under a project and I am in the project task list view, I am unable to pick people to assign tasks to.
The people list opens, but it is blank. I have actually found out that the list of people appears to float away from and below this box on a long list, hence it disappears out of the bottom of the window (I think).
When I filter the list (say by due date) down to a smaller list, I am able to see the list of people and assign the tasks.
If you would like a screenshot, please give me an email address to which I can send a snapshot with a bug report reference or something.