ok thanks. I was testing with my local install (which is our latest developer code) and I couldn't reproduce this. I also tested with one of my online installs and as you can see via this temporary example it doesn't reproduce the issue either even if each of the media items have been added without any media.
mitchellmedia.co.uk/xot/play_213#page1
However that install is the latest developer code too so I tested on another install (3.10x) where I can reproduce the issue. I checked our issues and commits and cant find any reference to this so I'll check with colleagues who have worked on this page type.
Meanwhile you could try replacing the timeline page in your install with the version from our developer code. Rename the attached timeline.txt file as timeline.html and replace the version you have in \modules\xerte\parent_templates\Nottingham\models_html5\timeline.html Make a backup copy of the existing file first because as I haven't found the issue or commit referring to this I can't be sure if there are other dependencies or not.
HTH
Ron