It happened to me several times yesterday, too - haven’t noticed it before, so maybe an unexpected result of the update? So far I’ve found that if I access an existing note and then come back to the new one it will generally work. It’s a bit of a distraction, but at least it gets me there in the meantime
P.S. - I think I’ve found a workaround - I put some data into an untitled note and added the title afterwards, and had no problem editing the text and adding more. Hopefully that will keep it under control till there’s an official fix