Thanks for your answer.
Unfortunately, nothing worked.
When I enter a new shortcut it lasts as long as I don't press the arrow in the top left corner. After I press it, when I come back to my keyboards settings, I have no more shortcuts entered.
You could always try restoring the iPhone and setting it up as a new phone.
Take a look at this Apple doc for a walkthrough -> iOS: How to back up your data and set up as a new device