It seems like that is bug, I will lock this issue into our issue tracker.

If you urgent to setup this, you have to manually edit through database this table #__discuss_post_types temporary, for example :

1. Bug
2. Issue
3. Task

If you would like to show the `task` show first, you have to update that `lft` and `rgt` columns value :

lft | rgt
----------
1. Bug - 3 4
2. Issue - 5 6
3. Task - 1 2