I noticed when creating a timeline with a large enough number of events (generally when you would be required to scroll down to see all the timeline) that the progress bars displayed for group or sections (when in closed summary state) don't always show the complete progress bar.

When near the bottom of the screen:

Scrolled down (about 2/3rds down the screen):

When near the top of the screen:

As you see, when near the bottom of the screen, only part of the progress bars are shown. At the top, the complete timeline is shown

I've tried to re-create a simplified example. In this example, I got four projects with the same event durations. There should be four separate progress bars for each summarised project. However, projects 3 and 4 are not shown all four. See below

If I scroll (with middle mouse wheel) so the projects are near the top of the screen, then all four events re-appear