We apologize for the issue you are experiencing after moving to our new Preview. Moving to the next message after a move or delete is a configurable option both in the old and the new experience. Is the behavior you are seeing now different than before you were moved to the new Preview?
You can change the option as follows:
1. Click on the gear menu in the top right and select "Options". This will take you to our Options page
2. On the left-hand side click "Message options" (under Mail -> Automatic processing)
3. Change "After moving or deleting an item" to be "Open next item"
4. Click the Save button near the top
5. In the left-hand pane click the <- Options button to go back to your mailbox
Please let us know if you are still having problems after this or if any of the above is unclear.