Hello,
Can you show the sort key defined for the G/L account in FS00? Did you try replicating the scenario? Is it happening for newly created documents also? If yes and PO number is populated through sort key, then you can check with SAP.
If new documents are coming fine, check table BSEG and field ZUONR for these documents to see if it is data issue or display issue. In any case, you can contact SAP.
Note: Please refrain from mentioning the words like it is very urgent. This is against SCN rules of engagement and moderator may lock your discussions.
Thanks,
V V