I discovered that today in version 1.0.71, it happens on both smartphone and tablet. The content isn't lost, if you tap on Add comment again, it reappears, only the cursor has moved to the beginning. When adding an answer, everything is ok.
One other thing on the smartphone, when you change orientation to landscape, the keyboard disappears too.
The tablet is a Nexus 10 with 5.1.1, the smartphone is a BQ Aquaris E6 with 4.4.2.