Gotcha, cool and thanks, @JGirault :)

@Mholloway, we discussed this in today's Discussion Meeting (which was a kind of maps triage meeting). As per the "Note" at the top in blue, what have you learned about this task? Will we plan to fix it or to warn users?

@JGirault's analysis is accurate, and I think fixing this is a complicated enough undertaking to be beyond the scope of Map Improvements 2018. That said, we could provide better error messages about what's going on, and update the docs to clarify the current inability to load chained ExternalData.

Updated the docs.

https://www.mediawiki.org/w/index.php?title=Help%3AExtension%3AKartographer&type=revision&diff=2820552&oldid=2813245

Regarding error messages, see T196015: Create an error message to better report about failings of maps with complex GeoJSON.

Are we sure this is just with wikidata ids etc ? I see something like this with even a simple .map page

<mapframe width=300 height=300 zoom=17>
{
  "type": "ExternalData",
  "service": "page",
  "title": "Haarlem/Grote Kerk.map"
}
</mapframe>

Rendered on nl.wp, as if it is located in Algeria instead of in Utrecht, Netherlands

@TheDJ It sounds like you are describing a different problem (and one I can't reproduce). Do you have a page published that demonstrates this?

To your broader point, I think you are correct that the problem in this task doesn't turn on the presence or absence of Wikidata IDs; I think this is a specific example of the general problem that the map loader can't handle chained ExternalData references, and if present they will prevent the map from loading at all.

@Mholloway we have since concluded that T193455: <mapframe>: Automatic zoom and centering only works with interactive maps, not static maps is a more likely match of the issue described.

Was it intended in the first place to support "data":{"type":"ExternalData"}? This seems to be undocumented on help page. Shouldn't <maplink/mapframe> and other data reusers just query OSM data and other map data pages directly?

Note that currently only 11 data pages (of which only 3 are not sandbox subpages) use ExternalData on Commons. So maybe it wouldn't be too difficult to disable this confusing "feature"?