That is normal. If you hit the unlock button and do not open the door, the car thinks you mistakenly unlocked the car and will lock the doors after a certain amount of time. Try it out and watch. Hit unlock and do not open a door. After 60 seconds, it will lock itself. If you leave it unlocked and don't press the unlock on your key fob, it'll stay unlocked. If you do press the unlock on the key fob, open the door and close it. The car will not lock itself afterwards. My Honda and Subaru does the same thing.
From page 113 of the owners manual:
Security feature:
If a door is not opened within approximately 60 seconds after the vehicle is unlocked, the security feature automatically locks the vehicle again