I was commenting that it looks strange because my usage drops so abruptly so I can’t see any real detail since 15 November. But the balloons are just as strange and it probably is a bug that the event balloons don’t appear on the entire graph. This problem has been reported before:
Like @csdfg2, I used to have the balloons appear for more of the graph when I restarted GlassWire but the balloons would quickly disappear for dates more than about six months ago.