From https://phabricator.wikimedia.org/T302861#8202688

geojson-mapnikify keeps its own cache of markers, in node's os.tmpdir()/geojson-mapnikify-<md5>.png. In order to refresh the snapshot icons, it's necessary to shut down the server, remove these files, and restart.

So fully-rendered pins are cached by geojson-mapnikify and stored in the /tmp directory. This would probably be much better to keep in-memory and include a TTL.