Office Word Online will send twice PutFile requests to WOPI Host when editing a document

Question

When I edited a document with Office Online Word, I found oos will send 2 PutFile requests to Wopi Host. Firstly, it will send a PutFile request when the status is Saving....Secondly, it will send a PutFile request when the status is Saved about 1 minute later. If I continue editing, it will send 2 PutFile requests. And seriously, the first Saved status document's MD5 is different from the second Saving... status document's. The Wopi Host will get 2 same-content documents with different MD5s, and generate repeat history versions.
Pls help me to solve the problem. Thanks.
Version: 16.0.10383.20001.