Hello lmaty,
Welcome to the Apple Support Communities. From your post, it appears mail messages are getting flagged again unexpectedly. We'll do what we can to help.
Since it's happening on two separate devices, that might be dealing with the network or the mail account being used. If it's the same account this is happening with, verify with the webmail if it's happening there. This is usually done through a browser and going to the website directly of your provider.
Otherwise, try using your iPhone 6s to get on a cellular connection and remove the flag to see if the same thing happens. If it doesn't, that means there's an issue with the connection on Wi-Fi with your Apple devices. Check the settings of your network: Recommended settings for Wi-Fi routers and access points
Thanks and have a great day.