Install files in subfolders

We can not install files in subfolders using one Group item.
For example, we have such folder structure:
Src
– Folder1
------ file1-1.txt
------ file1-2.txt
– Folder2
------ file2-1.txt
------ file2-2.txt
------ file2-3.txt
And we want install all files using one Group item:

Group {
        qbs.install: true
        files: ["*.txt"]
        prefix: "**/"
        qbs.installDir: "files"
    }

In result we get such file structure:
install-root
– files
------ file2-1.txt
------ file2-2.txt
------ file2-3.txt
I think it's not expected and incorrect result and it become to big problem if we using more subfolders.