As you can see in the screencast below, a new entry named "Fedora..." appears and the side bar is resized when the disc image is unmounted. Such behavior occurs randomly when I mount and unmount some ISOs in sequence. Nautilus shows this message when the behavior occurs:

** (nautilus:19785): WARNING **: 17:25:52.219: Failed to measure available space: Error getting filesystem info for /run/media/myuser/Fedora-WS-Live-28-1-1: No such file or directory

I use nautilus 3.28.1-784b3eaa on Arch Linux.