Numbering series in PO Draft change by itself when adding after approved

Dear Expert,

I am using SAP B1 9.0 PL 12. I have a problem with numbering series.

In PO Document i create multiple numbering series which is would be set according to vendor code when user input. (one of them set to default)

When user create new PO, i create fms to change numbering series according to vendor code. it's already working well.

And then the user adding the document, and the document automatically become draft because it need approval.

After the PO approved, the same user that create po want to add the document, but numbering series has changed again by itself to default numbering series. Not the same numbering series that set with fms when first input. And this numbering series can be changed (already gray).

Can anyone tell me what cause the problem.

Thanks

Yesaya