While Section cut is done through a diagonal Pipe, the Section view is coming as Rectangular lines instead of Elliptical.
We have so many Display option in 2D Display Settings in View > Object display in Detail Style.
When Display is switched to Outline, even though it is cut diagonally, it will give output as rectangular considering the extreme outline and it will look like as below
Here you can see the output is coming as Rectangular.
So to get the true cut, the Display to be switched to Standard which will give the correct cut as Elliptical as shown below.
So in this way you can get the correct graphic for the Pipe Section when cut diagonally.