When viewing current projects in the  Project Timeline Gantt view, it only shows a date range of current year, plus one year before and after. (E.g. if today is in 2018, then the Project Timeline Gantt view will only show 2017, 2018 and 2019.)

The problem is that any project with start or end date outside of this range (e.g. start before 1 Jan 2017 or end after 31 Dec 2019) will not be visible at all on the  Project Timeline Gantt view.

Even if the date range only shows 3 years, there is no good reason that projects with a long duration should be hidden from this view.