Flatpak version of QtCreator doesn't see system libraries

When trying to compile a project that uses system include files (located in /usr/include on my PC, such as newt.h) flatpak version of QtCreator doesn't find the included file, though QtCreator installed from the repository does find it and compiles the same project.

I like new versions of QtCreator, but I have to develop for an older platform (Ubuntu 20.04 and the derivatives), and neither newer versions of QtCreator installed via Qt Maintenance Tool work, nor the flatpak version does. The only version that works is the one installed from the repository.