Disappearing some SO backordered lines

Hi there,

I really hope you can help me with something unusual that has been happening here for the past month.

We've been experiencing some issues with SO backorders. Normally, when we print the Pick Ticket and there isn't a sufficient quantity, it backorders items. When the invoice is done, the Sales order still contains all the items that have not been fulfilled. However, with certain Sales Orders, when we invoice them, the lines that are partially fulfilled are being deleted, and only the ones where the backorder quantity is the same as the original quantity remain.

I've checked, and the items have the checkbox for Allow Backorders activated. So, I have no idea what's going on there.

Thank you in advance for any clues.