I tracked it down a bit, using the URLs
https://www.semantic-mediawiki.org/wiki/Help:Leaflet_format vs.
https://www.semantic-mediawiki.org/wiki/Help:Leaflet_format?debug=1
These weird "shadow" pointers are indeed shadows. The bug happens on
the "leaflet-shadow-pane" layer. The individual
"leaflet-marker-shadow" <img> elements on this layer are supposed to
show the image "marker-shadow.png", but for some reason show the image
"marker-icon.png" instead. This happens in the actual src="…"
argument
of the <img> tag, not in CSS.
There is really something broken there:
src="…/marker-icon.png?2273e")marker-shadow.png"
I was not yet able to track it down further.
The code your demo
https://github.com/JeroenDeDauw/MediaWiki/commit/1713ccde9de7d59634b1a134c5…
happens to touch does not look like it should affect HTML, but only
CSS. I was not able to find any code in ResourceLoader that messes
with <img> tags or src="…" arguments.
Best
Thiemo