I recently had a problem that sounds similar to yours. The drivers door would give the impression of being closed properly but would occasionally fly open on the first left hand bend. If, when I got in the car, I moved the inner door handle to the locked position it did seem to prevent the problem, but not a proper cure.
I lubricated all the mechanism and removed the door car to clean and lubricate the whole of the operating mechanism. There was old dried up grease and dirt. Since doing that, only a couple of weeks ago, I haven't had the problem, need to go around a few more bends at speed to be totally sure 😁.
Hope this is of some help to you.