Running a Qt application like OBS or a minimal main window (e.g. qt-wayland-adwaita) with "adwaita" decoration in a Flatpak with GNOME Builder.

qt-wayland-adwaita is a minimal reproducer that just enforces the "adwaita" decoration over a empty MainWindow.

Output found in OBS and reproducer:

qt.qpa.qwaylandadwaitadecoration: Failed to find an svg icon for  "window-close-symbolic.svg"
qt.qpa.qwaylandadwaitadecoration: Failed to find an svg icon for  "window-minimize-symbolic.svg"
qt.qpa.qwaylandadwaitadecoration: Failed to find an svg icon for  "window-maximize-symbolic.svg"
qt.qpa.qwaylandadwaitadecoration: Failed to find an svg icon for  "window-restore-symbolic.svg"

From what I read from the source code of the decoration, some assumptions were made where it should not.

It assumes that the application itself will provide an "icon theme" with needed icons, it assumes that the "fallback icon theme" will have them.
It even assumes that the Adwaita icons will be there where it is not a hard dependency.