AFAIK standard VAG behavior for auto locking is as follows..

If you unlock the vehicle but do not open any doors, it will relock after a time period. I can confirm this behavior on 2012 Passat, 2017 Passat, 2018 S6, and 2016 A6.

It behaves in this manner as a safety feature. In case you unknowingly unlock the vehicle with the fob (like you sat on it in your pocket) but never open the door (because you sat on it and weren't getting into the vehicle), it auto relock.