You will want to take a look at page 43 of the Notion for iOS User Guide. Here you will find the supported notation for the Handwriting App. The User Guide can be accessed by tapping the icon of the question mark at the bottom of the screen.
You can also try using the Handwriting window instead of writing directly into your score. This option is available by tapping the icon of the Handwriting Tool at the bottom, center, of the screen.
If the problem persists, please feel free to submit a support ticket from within your my.presonus.com account.