The reason is probably that you did s lot of changes, including groups and question types, during data colldction.
You may try:
- to download/export with option all versions included
- to temporarily go back to the previous version (clone XLSForm version from history view, download, upload/deploy) to download the older cases. (But you should stop data collection for a moment to avoid automatic download of this older version on local devices.) And you will need to update back to your latest version after this download.
