Yeah, I found it happens even on story mode, mostly when you frequently reload between different saves or switch between episodes. When I restart the game, sometimes the issue goes away and sometimes the reply box still remains empty.
Hard to say what can be the cause, maybe some cache issue within the app, something in my system, or even switching between windows and apps. Also it looks like it happens mainly when the other person sends you a lot of messages at once, especially with Izzy.
Also I made a save with such faulty dialog and when restarted the game, the issue was still there, but I was dumb to overwrite that save. If I manage to reproduce it again in the story, I'll provide a save file.