Because you have started the activity a week ago (168 hours), so you cannot see the fine details that are crammed to the end (at least I assume it is at the end). Use the Trim function and cut off the redundant part.
In your case, actually Garmin shows your pace and heart rate a straight line (or oblique line), instead of a flat line, namely what Garmin displayed is the average value with tread of pace and heart rate, instead of your real time ones. Why Garmin wasn't able to record the real time pace and heart rate? It's most likely because the time set on your Garmin watch is not the actual time.
