Customer's purchase order number wrong default value

I’m having issues with customers purchase order field in sales invoice doctype.
If the field is empty, default value in SQL table should be (NULL). In my case the field “po_no” in SQL table is not (NULL) but it is empty, although it’s default value should be (NULL). Was working fine before.


Another issue is that, when you try to enter customer’s purchase order in Sales invoice doctype and hit save, field is empty. It’s working fine with customer’s purchase order date field.

Anyone have an idea where to look to fix this issue?