Been wrestling with this same issue for some time, and finally found a workaround.
Using Xcode 7.2.
Not sure exactly why this is happening, but Over Current Context
is not an available option. This project was created some time ago, which I have a hunch could be related.

As a test, I created a brand new project, and Over Current Context
is an option.

Since deleting and recreating my existing project was out of the question, I ended up doing the following:
Set the Presentation
on the segue to Default
. This is preparation for the next step.
On the destination view controller scene, set Presentation
to Over Current Context
within the Attributes Inspector
.

I was able to successfully see the parent view controller underneath the semi-transparent modal.
Note:
I'm supporting iOS 8 and above, so I don't have to mess around with setting the modalPresentationStyle
on the parent vs. the child.
Failed attempts:
Setting modalPresentationStyle
to .OverCurrentContext
on the destination view controller in prepareForSegue
did not work for me, because the Storyboard setting seemed to override my code.
Setting Defines Context
and Provides Context
did not seem to make a difference. I'm guessing these could come into play for some app architectures, just not my in my particular project.