I was pointed out to the following broken thumbnail: https://upload.wikimedia.org/wikipedia/commons/thumb/1/1e/Zevenhuizen_Moerkapelle_wapen.svg/375px-Zevenhuizen_Moerkapelle_wapen.svg.png
The commons image page is: https://commons.wikimedia.org/wiki/File:Zevenhuizen_Moerkapelle_wapen.svg
As you can see (hopefully) is that the thumbnail is not completely generated, only the bottom part. Not sure if this is a one off, or a more general problem.
Upstream issue: https://gitlab.gnome.org/GNOME/librsvg/-/issues/683