Hi Michele,
To troubleshoot this issue, I would like to confirm the following things：
1. Does the issue occur in OWA or Outlook client?
2. Whether some customizations are made at Outlook client, such as local archive or run retention policy?
Based on the current situation, I suggest you try to find these email items via search bar in Outlook client or OWA. If the email still cannot be found, please refer to the following article to check whether they are in recoverable items in OWA:
Recover Deleted Items
Thanks,
Johnny Zhang