@joulesmc2 

I'm not sure how the HelioCampus Assessment integration is set-up/designed, but there is Canvas API support get submissions (which includes grade information) for concluded enrollments.  List submissions for multiple assignments 

If they are using an API Developer Key they could potentially have access to this API endpoint.  However, if they are only following the LTI1.3 standard, they wouldn't have direct access to this endpoint.  If this is the case, it may be due to limitations with the LTI standards, but perhaps someone with more knowledge about these could chime in and share how LTI's handle concluded students.

In the meantime, i would recommend continuing to work with the HelioCampus Assessment technical support to try to see if they can address this issue.  Since it is overwriting assessment data when an enrollment is no longer 'viewable' perhaps they could instead provide an option to users to control whether it should overwrite the last score or leave the last score when an enrollment is no longer found.  Alternatively, perhaps they could simply leave the previous score and include a warning indicator when an enrollment is no longer 'viewable' to their integration.  They can also contact the Instructure Partnership team if they are seeking additional support/advice on building/improving their integration to better handle concluded enrollments in Canvas.