I scrolled down to the end of a post already containing two images with the intent of adding a third. I looked for the editor toolbar so I could add media, but couldn’t find it. Turns out, it was hanging out at the top of the page, outside the editor. Scrolling down and then back up when a feature image is set reliably triggers this for me. Most of our themes encourage setting featured images. I use them often. This bug is part of my daily routine.
There’s a similar bug on iOS: