Thank you for escalating this problem.
The conflict is actually occuring due to the units of work. When you set the task's work to fixed and assign a duration and then assign the resource to the task, the units of work for that user are by default calculated as 8 hours per day. And according to this, the duration is recalculated.
However, our development team is working on it and we are in a process of making this feature more flexible in our further release.
Meanwhile, if there are some other concerns, Please feel free to contact us.
Happy Using Ganib.