Hi Mathis Viethen,
As per your description, it seems you use an Apple Pencil, to take advantage of iPad Scribble functionality to write text (without using the onscreen keyboard)
Please check if Scribble is enabled on your iPad (Settings> Apple Pencil and make sure that the switch next to Scribbleis enabled).
After search lot here I found article, you can refer and see if its help: How to Fix Scribble Not Working on iPad
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
If all setting it correct in your iPad OS, then try to perform reset OneNote troubleshooting steps for iPad and check it make any difference.
Reset OneNote on your iPad:(Before Reset OneNote for iPad please note make sure that all notebooks are synced or have backup in case of any data loss)
a. Ensure that the OneNote app is closed.
b. Go to Settings.
c. Find the OneNote app you want to reset.
d. Select the app.
e. Scroll down to reset OneNote app (Delete Sign-In Credentials and turn the toggle on).
Make sure OneNote is not running. (Double tap the home screen to see all running apps and swipe up on OneNote to close it.)
Re-open OneNote and you will have to sign in again. Once done, please check if you can use apple Scribble function in your OneNote.
Meanwhile, if you don’t mind, could you please confirm when this issue starts e.g., after updates iPad OS or OneNote version.
I appreciate your understanding and stay safe!!
Best Regards
Waqas Muhammad