DUB: Sometimes generates .di file and sometimes do not.

[BoQsc via Digitalmars-d-learn]

I would like to know how to solve this problem with `dub`.
**dub.sdl**

```
name "dheaders"
description "generates .di header file for a static library."
authors "public domain"
copyright "Public Domain. No rights reserved."
license "public domain"

configuration "staticLibrary" {
        dflags "-Hf=$PACKAGE_DIR/builds/library.di"
        targetName "library"
        targetType "library"
        targetPath "builds"
        sourceFiles "library.d"       
}
```

**library.d**

```
module library;

int func(int x)
{
    return x+1;
}
```

**Command Line:**
```
dub
```

**Problem:**
After running `dub` command: `library.di` file is 
rarely/inconsistenly generated inside `builds` directory.