Having same problems this week. Note contents dissapears or even is replaced by content of another note! Just now checked again and found more notes with such problem - missing completely empty content. Some of them has content still available on EN website, however on PC content is missing. For example, I select a note having this issue, do "Copy internal link" and paste into the browser and then I see the contents. Note creation and update times are exactly the same back from beginning of 2017 and its simply a webpage clip from Evernote Chrome extension. So I am 100% sure never touched it or edited. I already opened a ticket in support. Will see how the problem will be solved.
Activating the Premium account with note history feature is not an option, because its definetely a bug. If the note content is missing, it will not appear in the results when I do search in all notebooks and find notes in my archive having some particular word mentioned. It simply will not show up anymore as content is missing and I will not notice it.