PO line sequence resets on form save after line changes

[dangirasv]

module: purchase_order_line_sequence
version: 14.0

Steps to reproduce
Open already created PO record with multiple lines. Edit the order and change the order of PO lines. Sequence numbers will change live depending on PO line positions, but when saving the record, the new sequence numbers are reset and the lines are back in their original positions.

Current behavior
PO line sequence number (and positional) changes are not saved after edit changes.

Expected behavior
PO line sequence number (and positional) changes are saved after edit changes.