Just upgraded to Nougat - Textra notifications disappearing from tray
I had the same problem. The Textra app includes a support section (settings > support) and this is one of the top topics. Here is the text.

OnePlus running Android 7 - Notification disappears / goes away

To fix, do Android Settings > Battery > Aggressive doze and app hibernation (dotdotdot top right) > set Textra to 'Not optimized'.