QtCreator 4.3.0 (clang backend) and higher cannot detect our include directores for Qt and sometimes internal headers with a cmake project.

Looks like the cmake server mode does not provide enough information or QtCreator does not ask everything it needs to know.

You can try our project by yourself:
https://github.com/Governikus/AusweisApp2/

Maybe QtCreator does not add IMPORTED_TARGETS or includes for an INTERFACE?

TARGET_INCLUDE_DIRECTORIES(${_name} INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>")