As Microsoft just recently applied the latest update 2020 wave 1 to our instances, which includes an update to the editor to the notes section. Previously, it was using a plain text editor where spelling checker from the browser can interact with it.
With the new rich text editor, when you right click the incorrect text it will now showing the cut/copy/paste options instead.
To use the native browser’s spelling checker, now we need to use CTRL + Click
Hopefully this can help you with the usability of the new control that is available.
Credit to Elaiza Benitez, who pointed out and send the workaround.