Issue with install prefix

Hello guys,

Please excuse the non-telling subject of this e-mail. I am a bit lost here and don't exactly understand where the problem I'm about to outline originates form.

A few weeks ago, I created a port of a C++17 library which is built using CMake. Everything went very smoothly.
Now I wanted to create another port, also of a C++17 library which is built using CMake. While the port itself compiles & installs successfully, I cannot pass `make check-plist`.

What I basically see happening is that the port builds and installs correctly but it installs under /usr/local/ rather than in $STAGEDIR/usr/local/

The only notable difference between this port and the one I already published is that this one depends on Qt5 libraries. However, I doubt that this has an effect.

Building & installing the library manually (outside of FreeBSD's port infrastructure) allows me to control where the various files are being installed by CMake by setting CMAKE_INSTALL_PREFIX.
This leads me to believe that there is nothing inherently wrong with the upstream CMake scripts.

I'm not really sure what to ask for other than: Where do I go from here?