As far as I know there shouldn't be an auto-deletion of sent surveys unless the inbox is enabled as a survey response can only exist in one folder at a time. I would suggest that the users 'fix' their database. On their device they can click the upper right corner to open the menu, then 'Settings' --> 'Storage' --> 'Fix Database'.
Also, we have seen a few instances where survey responses were not be visible in Survey123. We found that enabling the overview folder has helped to find survey responses which are otherwise 'missing' from the drafts and sent folder. The "enable the overview folder" option in Survey123 Connect is under the 'Options' --> 'General' section. See more info here: Publish a survey—ArcGIS Survey123 | Documentation
Farley