Thanks for the details.
I have found the cause. It was suspicious that some results with the taxonomy term were present, when others don't. When I opened the post in question and save it without any changes, it instantly appeared in the results list – meaning it was not indexed.
Then I went to the index table page and re-created the index, and indeed there were missing elements, as the keywords count went up to 91k from 56k. I guess the posts were duplicated for translation after the index table was created, so they were missing from the index table. The duplicate process does not automatically trigger indexing, as it would probably cause a timeout error.
Only saving/editing/deleting posts update the table automatically, as individual items does not take too much CPU to index.
Also, the limit to the results is set to 10 – so the item might not appear, but I think you can safely increas it to 50, based on your server performance.
In case you plan to import/duplicate large amounts of items, I suggest re-creating the index table after, to make sure newly created elements are indexed properly.
I hope this solved the problem!Best,
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