I created a new task template but no milestone display in the default summary view drop down box when it is used. All milestones are selectable when 'all options' is used from the default view. I am guessing that a milestone has to be included in the default task template. However, I am not sure that makes sense given milestones have a selection option from the edit summary page when the template is used.
Update: It seems sometimes the task template does show some milestones, but not all of them and not most of the time. I have yet to identify what is different that may account for such inconsistent behaviour.
