@PinkElephant @Dave-in-Decatur thank you both for replying to my issue. The strange thing was actually that somehow the content of another (second) note replaced the content of my initial note. Out of nowhere and without syncing. It even happened for 2 other notes as well, where the content of one note was copied automatically to another note. Even more strange was that I didn't even work in those notes that day.
Long story short, I went for the subscription for a month and restored the older versions. So problem fixed, but the cause is still unknown.