Hi mav
I am not telling pressure force will not be there in bellow portion. I am telling for pressure balance bellow modelling purpose consider thrust force zero at bellow portion.
Pressure thrust force is always there in pipe & that creates pressure component of longitudinal stress.
In a continuous pipe you will get component of longitudinal stress ( internal force due to pressure) but no external force.

Whenever we are using untied bellow then only we get unbalance pressure thrust force in external reference like support equipment otherwise not.
Since pressure balance bellow act like a continuous pipe as far as pressure thrust force is concern. So we can ignore external pressure force on both side of bellow( since it is balance one).

Expert please correct me if I miss some point.