I'd be glad to help you show invoice notes in your QuickBooks Online (QBO).
I've replicated this on my end just to make sure and it works fine. In your case, your browser's cache may cause this unexpected product behavior.
To resolve this, we'll have to perform some of the basic troubleshooting steps. Let's start by using a private browser to isolate the issue. The steps vary depending on your type of browser. You can follow the keyboard shortcuts provided below:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift + N
Now try creating an invoice and see if the message show up in the customer note field. If it does, you can switch back to the regular browser (Ctrl + N) then clear your browser's cache.