In the project description, the addings of "TG and phase" is completely independant from the milestones defined in the project tasks.
There should be an option when entering a new milestone in a task in order to raise the need to bring this milestone dates as a major step for the project, and so be included in the overview.

With this functionality, it will not be necessary to enter twice the information in tasks and project overview. Furthermore, when the milestones are delayed it will refresh automatically the overview dates.