Hi Irwan,
I dont think that there is a way to track why the link was broken but could be due to:
1. User used copy from GRPO
2. Changed the vendor
3. Selected the same vendor again and added the document.
This would not reflect in the change log as the change log would show changes only once the document is adde and not the changes done before the invoice was added.
You may opena ticket with SAP to investigate further.
Thanks,
Joseph