Hi @NoelleHoenig ,
On the Turnitin Integrations Known Issues page, there is an entry titled: The submission date and time updates when an assignment is graded. (I'd link direct to the entry, but their anchor links don't seem to be working correctly.)
We have identified an issue where the Turnitin submission date and time displayed within Canvas will update when the assignment is graded, rather than retaining the original submission time and date. Turnitin and Instructure are exploring a fix for this issue.
Could this be the cause of the issue your instructor is facing?
On further reading though, I think your case sounds slightly different. You may want to contact Canvas and Turnitin support for them to follow up on this.