frappe.MappingMissmatchError throw message is not stable

Hi everyone, Does any one help me out why this frappe.throw(_(“Qty for row {0} does not match Material Request”).format(item.idx),frappe.MappingMismatchError)
message is not shown in UI. Sometimes the message is displayed on UI and sometimes it not getting displayed in console msgprint is diaplayed along with traceback message.