The "Vendor" field in a Purchase Order cannot be edited / updated. Admin users also cannot edit/update the value on "Vendor" field.
1. The Out of the Box write ACL on proc_po.vendor is restricting the write access on the column , if the record is not in "Requested" status.
Review > https://<instance-name>.service-now.com/sys_security_acl_list.do?sysparm_query=operation%3Dwrite%5Ename%3Dproc_po.vendor
2. The ACL , has admin override to false , hence it will also not allow the admin users to write on the column if it does not satisfy the condition.
This is working as designed.