Hi Dave,
Since the user can be shown properly in a new distribution group (DG), it turns out that there is something wrong with the original DG (which was converted from a SG).
Based on this situation, I suggest you export the members list of the original DG, then delete the DG and recreate it. To export the members list, you can run the following command in Windows Powershell:
Get-DistributionGroupMember –Identity DGAddress | export-csv D:\export.csv
The file export.csv can be found in D drive.
Thanks,
Mindy Pan