When you flip your iPhone over to the landscape view when you’re typing a message, it automatically changes to the handwriting interface. Chances are, most of us don’t handwrite messages and would prefer the landscape keyboard here. Good news, though, How-To Geek points out that when you tap the keyboard button, iOS remembers your preference.
When you type out a message in Messages and flip your phone horizontally, just tap the keyboard button in the bottom right corner. That pulls up the landscape keyboard. More importantly, your iPhone remembers this choice, so every time you rotate it in the future, it will continue to display the keyboard instead of the handwriting page unless you change it.
How to Turn Off the Handwriting Interface in iMessage | How-To Geek