I cannot reproduce.
Are you sure that both tasks are "Fix duration" ?

Please try and reproduce on Demo.

I cannot reproduce too.
But on this project I cannot explain why it is doing this.
Have you a way to have a look on our ProjeQtOr instance ?
The Project is the following, and the two activities are at the bottom of the screenshot.

Thank you

It seems the parent activity 3.7.8 has a dependency with upper activity 3.7.7.
As it it drawn, I guess it is a end-end dependency.
Can you try and remove it ?

Thank you that solved the issue.

But I don't understand why it works like this ?
Because for me this dependency is a constraint which prevent 3.7.8 to finish before 3.7.7. So I dont understand why it has an impact on this.

Thank you

Hi,

It is a restriction of ProjeQtOr, which does not really manage "backward" planning.
When you set an end-end dependency from predecessor X to a parent activity P, all the activities (A and B) on the parent get the dependency, and this will "remove" internal end-start dependencies.
What you may do is define end-end dependency from prececesor to last activity under the parent activity (from X to B).
But I gess you won't really get what you expect as predecessor of last actity (A) will be planned ASAP, and not The Latest possible to fit the end-start dependency. 
This is because of unavalability of "reverse planning mode" (what you expect is some reverse planning inside the parent activity)

Hi,
Ok thank you for your help.