Searching multiple words gives less results than it used to

So, following the documentation for this situation from https://extensions.xwiki.org/xwiki/bin/view/Extension/Solr%20Search%20Application#HSearchSyntax , I am doing the following:

• searching for a word A ("essai" in my case)
• I have 3 results
• searching for a word B ("Unicorns" in my case)
• I have 1 result
• now, what I want to obtain is search results that match either Unicorns or essai; I am expecting at least 3 results, maybe 4 if there is none of the result that matches both. I have tried the following:
• "Unicorns essai" , without the quotes. This doesn't work, I only get 1 result, and it's normal, it's what the documentation says
• "Unicorns OR essai", without the quotes. This doesn't work either, I also only get 1 result, it's normal, it's also what the documentation says
• "+(Unicorns essai)", without the quotes. This also returns only one result, while the documentation says that this should have the result of the "real OR" .

I can reproduce this on XWiki version 16.10.2 and XWiki 17.10.2 .

Now, maybe it has an impact that my content is in a multitude of languages, more or less correctly indexed (they are impacted by XWIKI-23999 ). But even so, regardless of why the en_GB results are not found for all the searches (in my case they are not found for the search on Unicorns but they are found for the search on essai), I am expecting to be able to combine the results of 2 searches with some sort of syntax...