3820020 - Why "Request Save" audit rule triggered on submitted request while approving?

Our company use one audit rule to monitor overdue 30 days request by calculating field "Start Date" on request header. This audit rule trigger event is "Request Save". However, we faced one issue that when user submit this request, this audit rule not triggered, instead it triggered while approver approving it, and the approver could successfully approve. The exception generated After approving that block the next approver approval. May I know why this happened?

"Request Save" audit rule triggered on submitted request while approving. , KBA , CON-CA-ADM , Administration , Problem