Outlook 2016 has a new "feature", one that will automatically purge items from your folders if you are using an IMAP account. Great if like and empty mailbox, not so great if you use you don't want to delete anything.
The best advice is to not use the deleted items folder as a storage location for emails you wish to keep. However if you must then there is an easy fix for this:
1. Go to your accounts settings and change your account
2. Click on the more settings button
3. Select the Advanced tab.
4. Clear the check mark in the last box marked "Purge items when switching folders while online"
Once you've done this, you can still purge any folder manually.