The same thing happens to me. In this case, I think it is a synchronization problem.
I write something in a note, go to edit another note, and when I go back to the previous note I see that what I wrote was not saved.
I am accessing Evernote from Chrome. When I try to exit Evenrote, I get a message telling me that the sync was not done. I wait several minutes to see if it synced, but I get the same message.