On Android, certain apps can override any user settings and “force” the keyboard to go fullscreen. This means you cannot see any context of what you are replying to.
The two versions are as follows:
1. Why would a messaging app such as WhatsApp want to do this by default?
2. Why would any app want to do this?
(I see you have a larger area to compose but is that actually useful? It’s easier to remember what you’ve typed yourself. Wouldn’t portrait be better? Or making it an option?)