I have seen countless videos, including that from developers, where they are running forward and able to check their inventory without stopping. I had never been able to do this. I play lefty and so I changed all the key bindings to fit my play style. Whenever I am on the move and check inventory, the character stops moving forward. It makes no difference whether I am walking, running, or sprinting.
UPDATE: based on user vatixerid's information, this has been confirmed to be an issue with the UP ARROW KEY. If pressed while checking inventory on the move, your character stops moving.