Parser doesn't find header files - cmake compiles fine

Qt Creator does not find the header files in a header-only library.

A simplified project is attached below (or https://github.com/bfrg/qtcreator-test). A custom target 'headers' is added to cmake. When this is done, the header files are listed in the "Project view" but when one of the header files is opened, the parser doesn't find them (see screenshot below).

When the custom target is removed, everything seems to work. Though, the header files are listed in a different subdirectory (called <Headers>) in Qt Creator's "Project view" (see second screenshot).

Tested with Qt Creator 4.4.1 and CMake 3.10.0.