> Is this a bug of the tool or there are some settings to be changed?

That's a bug in the actual GFX driver that can occur with 64 MiB VRAM Mac minis and will be fixed in the next MorphOS release according to bigfoot. The GraphicBoards tool is always showing the VRAM amount MorphOS can access. So it's not just a wrong number in the tool, unfortunately. I have the same problem when autobooting MorphOS. When starting MorphOS from the graphical boot menu it shows full 64 MiB VRAM.