I have had a similar issue. I was actually trying both QtVirtualKeyboard (for Qt12.5.1) and Mockup to get an idea how these work, and what I would need to modify for my needs. What I discovered is that the QtVirtualKeyboard had an interface of "org.qt-project.Qt.QPlatformInputContextFactoryInterface.5.1" , while mockup had an interface of "org.qt-project.Qt.QPlatformInputContextFactoryInterface". My guess is that at some point the interface was reved, and Mockup is now a too old interface. Why I adjust Mockup to match QtVirtualKeyboard, it worked.
As a side note, when I run designer, neither shows up in the plugins dialog, even they are active (as proven by checking the filter box).