When I enable Dark Mode and or Sepia Mode a white rectangular box appears on the screen

This issue seems related with a bug in iOS 12 and it happens only if you are in dark or sepia mode. We hope that Apple will fix it in the next iOS updates.

 

Meanwhile, you can fix it easily: When you create a new blank text document and you are using dark or sepia paper mode, insert several blank lines in the note (tap several times the Return key).  After that, you can type in those lines without those weird visual issues.