Hey there Oceama,
Thanks for keeping us updated. It sounds like when you're viewing your unread emails by viewing them via the filter option, your emails disappear after opening them for a few seconds. If so, we'd like to recommend some steps to see if we can get this issue sorted out. Follow these steps below:
1.) Close the mail app.
2.) Refresh things by restarting your iPhone.
3.) If you have any pending updates available for your iPhone, please install them. This can usually help with unique issues, since updates include fixes. Just in case, this link can guide you through the update process: Update your iPhone or iPad - Apple Support
1.)Back up your device using iCloud or your computer.
2.)Plug your device into power and connect to the internet with Wi-Fi.
3.)Go to Settings > General, then tap Software Update.
4.) If you see more than one software update option available, choose the one that you want to install.
5.) Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.
If these steps don't help resolve your issue, please contact Apple Support for help. They might have to investigate this issue further with you. Use this link to reach them: Get Support
Thanks for using Apple Support Communities. Take care.