Hi @MarinJohnson ...
This would be news to me if documents from OneDrive would be able to be submitted via Turnitin. I do know there is a LTI external tool integration for OneDrive and another for the Turnitin LTI, but I'm not aware that those two external tools "talk" to each other in the way you describe. In my past role as a Canvas administrator in higher education, we had Turnitin integrated into our Canvas environment, but I don't recall ever seeing any functionality that would allow submissions from OneDrive. Students would always need to upload a document to the Turnitin interface that was saved on their computer hard drive. So, what you describe...
"When a student adds a submission via OneDrive LTI integration a similarity report is not created in the SpeedGrader. If they upload the file directly then a report is created."
...does sound correct to me.
I know this might not be the answer you were looking for, but I hope that it helps to answer your question. Let Community members know if we can be of any further assistance.