Upvote:5
What you're seeing is expected, and is due to the way the data is collected, and the difference in definition between an "arrival" and a "departure" as far as the CBP's Travel History site is concerned.
Departure - The US doesn't generally have any physical immigration on departure, so this data is collected from other sources. When leaving by air, this information comes from the airline. When leaving by land to Canada, the information is provided to the US by Canada after you pass through Canadian immigration. Thus every departure is recorded on this site.
Arrival - Despite using the term "Arrival" this site doesn't actually track arrivals, it tracks you being given an I-94 (or equivalent) when entering the US. For a new arrival, or most arrivals by air, you'll be given a new I-94, so the entry will be recorded.
However for short trips to Canada, it is possible to re-enter the US without having a new I-94 issued, using a process called Automatic Revalidation (there is a similar process for people from Visa Waiver Program countries that works basically the same). When you enter using Automatic Revalididation, your existing I-94 continues to be valid - you will not be given a new one, your "value until" status data won't change, and you will not have a new entry to the US recorded. In effect, it's as if you never left the US.
Thus you end up with the exact situation you have - the departure is recorded, but no new arrival shows up as no new I-94 was issued. This will cause no issues at all when re-entering the US at a later date - whether that subsequent entry is by land or air.