I am finding inconsistencies in the results reported by different wikipedias and between API and pages in the same one.
This search
reports 6 results
while the equivalent one
returns none even though this page https://hr.wikipedia.org/wiki/Klis has geographical coordinates, and should be returned by the above search.
Is it possible that geosearch isn't activated on some of the wikipedias - in this case the Croatian one?
The geosearch on Wikipedia is rather unreliable because coordinates can be stored in many different ways in an article. Although it seems that it works better on the English Wikipedia than on the Croatian one, also your query result of the English Wikipedia is not complete. For example, it does not include https://en.wikipedia.org/wiki/Kamen,_Croatia and https://en.wikipedia.org/wiki/Mravince even though these two places are located within your bounding box.
For doing geosearch, I suggest to use the Wikidata query service instead of the Mediawiki API. The following query returns you all places within the bounding box with an article on Croation Wikipedia: https://query.wikidata.org/#SELECT%20%2a%20WHERE%20%7B%0A%20%20SERVICE%20wikibase%3Abox%20%7B%0A%20%20%20%20%3Fplace%20wdt%3AP625%20%3Flocation%20.%0A%20%20%20%20bd%3AserviceParam%20wikibase%3AcornerWest%20%22Point%2816.456582641601567%2043.60834750781285%29%22%5E%5Egeo%3AwktLiteral%20.%0A%20%20%20%20bd%3AserviceParam%20wikibase%3AcornerEast%20%22Point%2816.55658264160157%2043.50834750781285%29%22%5E%5Egeo%3AwktLiteral%20.%0A%20%20%7D%0A%20%20%3Farticle%20schema%3Aabout%20%3Fplace%20.%0A%20%20%3Farticle%20schema%3AisPartOf%20%3Chttps%3A%2F%2Fhr.wikipedia.org%2F%3E%20.%0A%7D
Note however, that this query does not search coordinates stored on Wikipedia but on Wikidata.