Isx15 metal shavings in fuel filter

[Samvelm83]

Guys I'm confused. This is already 3rd PM when I get metal shavings in the main fuel filter (water separator - clean).
Pulled fuel pump head: Plungers, cam, tappets are ok. No signs of wear. Also removed low pressure pump. Everything looks good.
I do see shavings in the fuel tanks but why the water separator filter doesn't catch them which is the very first filter from tanks?
What is located in between the main filter and water separator filter that can cause brass shavings?