In OWA for iPhone if you have the email view set to Conversation View and then you search your email, the search results only include the most recent email in a conversation rather than all emails that include the search term. The search results need to either include all emails individually, or group them by conversation with a way to expand them like you have in the non-search view. If you have your email sorted by Date Received, you will see all of the emails - it is only Conversation View that is hiding relevant messages...