In my case, a note contained lots of screenshots to the point that it would take 10-20 seconds to display content. I had to split content into two notes and my empty picture was back to normal on reopening.
Apparently, there is an issue with Snipping tool. I fixed my issue by using Snip&Sketch tool that has been introduced in the latest update of Windows 10. Also, you can try to use third-party screenshot tools or simply save you screenshot in JMPEG first and then add into your notes.