On my iPhone, I responded to a message on mobile several days ago by tapping the notification on the lock screen, then most likely went back to the Home Screen with the chat still open. Since then, every time I open the app, it automatically opens this chat. I tried returning to the main chat tab or list of friends before going back to the main list of apps every time, but it never sticks. The only fix was to force close and reopen the app. It’s an easy fix, but in case there is some odd behavior you need to look at hopefully that helps. I don’t recall this issue happening before.