Email search results and selected email go away when navigating to another app

Hello, when I do an email search and find the email and select it (open it up). Then I click a link in the email which takes me to Safari (for example to get tracking info on a package). When I go back to the email app the email is gone and the app displays my same search string I originally entered in the search box however the results below now yields “No Results Found”. You have to go and perform your entire search again which sometime could be lengthy. This happens with every email box, I have removed and reinstalled boxes, I have the latest update on my iPhone 6SPlus and it happens on my husbands iPhone 7 as well. This has happened since I can remember using iPhones.