I costumized my startup file by appending a collection of sculpting-brushes from another file. All the brushes, textures, and images have fake-users to prevent them from being deleted and are packed into the blend-file.
However if I start blender via the desktop shortcut, it fails to load the textures for every custom brush. The references to images/textures are still there as well as the functionality of the brushes, but the images/textures are black and the Properties->Texture tab shows "Can't Load Image" for every one of them.
If I manually open the very same startup file located at %appdata%\Blender Foundation\Blender\2.80\config\startup.blend, everything works like a charm. Since the purpose of the startup file is defeated by having to load it manually everytime for it to work, I'm looking for a solution. Could this be a bug?