This has happened twice and the only common aspect I've noticed is that I had no internet connection and no cell service in a particular place.
I opened a note I had already created. I added some content, not thinking about how there was no cell service there (or spotty service). It gives me an error message "Unable to save note" (or upload content). That makes sense and I would assume that it would upload as soon as I reached a place with good cell service or internet access, just like it works when you're in airplane mode.
However twice after getting back to another location when I go to open that note it says it's locked and it displays the message...
"this note is read-only it was created in another app"
This is untrue. Twice the same thing happened and both times the notes were created on my phone and then I was trying to edit them again on my phone (but in the location with no cell service).
I've looked at the Forum before and can't see the same problem.
This seems to be a bug. As I said it should work like airplane mode, and upload once you have service again, instead of turning it into a locked read-only file so that you can never edit the same note again. I tried to create a copy of the note and it made that one locked also.
Would be great if they fix this problem.