Broken references in forming tools

I have a forming tool that I use frequently in our sheet metal parts. I normally check Link to form tool option in its property sheet while adding it.

Sometimes (though not always) I see a question mark in front of this forming tool in feature tree.

That obviously shows some external references are missing. But when I right click it and select list external Refs.... nothing is missing.

The attached assembly has a virtual part (01) that shows this problem.

The form tool is attached as well.

How can I fix these missing links or prevent my assemblies showing these question marks in feature tree.

Using :

Windows 10 with latest updates

Solidworks 2018 Sp0.1

Forming tool is placed in a local folder (D:\SolidWorks\Docs\My Design Library\Forming Tools)

Any kind of help is appreciated.