I have now found with a couple of Locks that when you successfully bumped the lock open  The cam does not engage and so therefore you cannot unlock the door.
There seems to be a trait with the Brisant E Locks , as well as the Zoo VS or V5 cylinder and more recently the format Cylinders, all of which can either be the thumb version or the Key/key cylinder as it makes no difference.
In this short and real world, therefore not all fancy and shiny and efficiently. Effective video. It shows you what happens and how it happens and explains how you can overcome this problem when in a real life situation at somebody's door where you need to get them in the house.
As always, my videos are a bit rough and rugged because I am working in the real world and want to bust the myths and give you solutions as opposed to showing you the nice pretty shiny, perfect practised version of doing things.