track always has id="overlay_track". Grepping the source revealed that this is created by this feature:

https://thediveo-e.blogspot.com/2016/03/kdenlive-ui-split-view-odds-and-ends.html

which with hindsight I realise is exactly what I was using each time it broke. So this feature is definitely broken, at least in 18.04.3.

More details see Bug 400660