After some investigation, I may have a clue of what is going on. Searching for "Marie Curie person" to try to make the Marie curie result to show in the results, the results finally shows but it is labelled as "Q7186".

Looking at the Wikidata item I noticed that it does not have an explicit label in English, because it is using the "default for all languages" value instead.

This is a recommended practice from the Wikidata community to avoid repeating the exact same label across many languages (especially relevant for people and Companies). We may want to make sure we support the multilingual label unless it is overwritten by a local one. For searching we could use both. For displaying, we may want to use the local version and fall back to the multilingual one.

@Pginer-WMF switching to a different search API here would allow getting "Marie Curie" (the scientist) as the first result in the search but it would mean undoing the work done for T422854: Make Wikidata search more flexible in Article Guidance.

We could technically use both search APIs and combine the results but there is already a lot going on in the search flow so I would prefer to avoid that if we can.

Correct handling of the language fallback, OTOH, is a simple fix.

Change #1279446 had a related patch set uploaded (by Sbisson; author: Sbisson):

[mediawiki/extensions/ArticleGuidance@master] wbgetentities: use languagefallback=1

https://gerrit.wikimedia.org/r/1279446

Change #1279446 merged by jenkins-bot:

[mediawiki/extensions/ArticleGuidance@master] wbgetentities: use languagefallback=1

https://gerrit.wikimedia.org/r/1279446

Currently on Test wikipedia the search results seem to work well for the examples of both cases (this ticket, and T422854):

Marie Curie	Titanic	Titanic film	Titanic (film)