This happened to me 2 months ago. I had written hours of notes and showed as synced (Mac 10.14.2 Mojave) and they even showed the preview when I navigated to a different note. Everything seemed good. The next day I opened the notes and several paragraphs (hours of typing) were missing, as if the last several saves of the document didn't fully save. I tried to contact customer support, but you can only do so with a Premium membership, which was enraging. So I signed up for Premium temporarily and still wasn't able to contact customer support. More enraged that on top of my first bug there was a second bug. Finally the membership unlocked the ability to contact customer support so I sent an email to a person who reached out to me weeks later. At this point I had given up, written off Evernote, and switched to Google Docs.
I've been using Evernote as long as it's been around. This was the first issue I'd ever had with it but it was so big, so frustrating, and so unresolved that I decided I was done with the product. My confidence in its single, most important purpose was majorly eroded. Hopefully this bug gets fixed, but looks like it's not only still a bug but not much public awareness from the Evernote team fixing or updating the bug.