I believe its the onscreen phone keyboard that causes the issue. It happens to me, I also have emoji and SimpleMDE extensions.
I will try to isolate the extension. However, for me, I just hit the Androids back button to make to the keyboard disappear if I need to review my text, but typically I just type away.
Edit, I am unable to reproduce the issue at this time. I am using an Android S5 to test, it might be useful to also mention the keyboard app used on the phone? I am using NextApp Keyboard. I have SimpleMDE and Emoji and Upload extension enabled.
I did notice OP behavior prior, not sure if its related to the browser on my phone or not. I will update this post when I find out more.