For some of my activities, Garmin Connect shows missing data in graphs when I choose time on the x-axis. This is the case in both a web browser and the GC mobile app. It looks like this:

I am guessing the cause is a faulty time registration/display: the x-axis suggests that I was riding for almost 7 hours, while in fact I only cycled for about half of that (3:45hs). What can I say, I'm a lazy guy ;-)
When I put distance on the x-axis, things make more sense: data are showing and the distance on the x-axis is consistent with the distance I actually covered:

What could be causing this issue? And, more importantly: how can I fix this? I ride a Garmin Edge 820 with up-to-date firmware.
*EDIT*
Now that I compare both screenshots: the graphs appear to be identical, apart from the empty ‘intro’ that has been added to the graphs with time on the horizontal axis. The section with data on it is equal to the time I have actually cycled. So it’s just a bogus intro that has been added to the graphs.
The question, however, remains the same: how to solve this?