Win XPx64 SP3, Core i7. Gantt Project 2.0.10, JRE version 6 update 22 (64 bit) & JDK SE 6 update 22 (64 bit) & Java FX 1.2 & some older JRE's are installed as well (update 18) and 32 bit update 22.
It's doing this crazy thing where, I can set any duration for the first task, but, when I create the first subtask (which is 1 day by default), it resets the first task duration to 1 day. I can then increase the duration of the second task (to say, 40 days) and then I can set the first task to the proper duration. Subsequent subtasks don't seem to affect the first task, and, they should. For example, if a subsequent subtask is 50 days and the original containing task is 40 days, it should update the containing task to 50 days. This does not happen.
To summarize, the first subtask is improperly affecting the containing task. The next subtask doesn't update the duration of the containing task, and, it should.