I've installed using the Qt online installer (qt-unified-linux-x64-3.1.1-online.run) Qt 5.14.0 (I will attach the screenshot with packages selected).
I've changed the default directory to a different one (in one case /opt/Qt in one /home/fede/Qt).
In both cases after seemingly successful installation I can see that the pkgconfig files installed do not have the correct installation prefix but seemingly a hardcoded one:
This of course prevents any tool using pkg-config to use the libraries. I'd expect (as so it was in previous versions) to have the prefix have been corrected by the installer.
As additional information: I did this on a CentOS 7.4 system and on a Fedora 30, with same results.