I was using the suggested workaround of adding the image I want for the thumbnail first and then waiting until it is "accepted" before adding any other images.
This worked fine (a bit annoying as I had to recreate a bunch of notes) up until the latest release on Windows 10.44.8.
Lo and behold, with no warning, not only does the first image in a new note not stay as the thumbnail as soon as another image is added, all of my old carefully created notes reverted to showing a different thumbnail.
Absolutely fantastic, So not only after 11 years of requests have EN not fixed the issue, they have now made it worse! Congratulations!