That’s odd! From the screenshots on StackOverflow it definitely looks like the presented (light blue)view controller is being shown as a card. The dark blue is probably coming from the main window’s background colour (and black might be a better choice for that).
I quickly created a test project in Xcode 11 beta 4 with a button that presents a modal segue and it worked fine.
Put a breakpoint in the presenting view controller’s viewWillDisappear. Does it get hit when you present the modal? If so, something is definitely removing it. Are you using a custom transition delegate which removes the presenting view controller? If it is safe to do so without risking breaking something else, can you delete and re-create the segue? What do you see in the Xcode view debugger after the presentation? Is the presenting view controller still in the hierarchy? If so, what happened to its view?
Sorry I couldn’t be more help. I hope you figure it out.