Reported here: https://www.mediawiki.org/wiki/Topic:Udbfgrmh3li6abqg
When searching within talk namespaces, the displayed search results are not in the selected talk namespaces.
In the example of the report, 125 search results were displayed, but all in namespace 14, that was not selected.
This is the example search provided by the user: https://cs.wikipedia.org/w/?title=Special:Search&fulltext=1&ns3=1&ns5=1&ns11=1&ns15=1&search=kategorie:Údržba:Wikidata&advancedSearch-current={"namespaces":["3","5","11","15"]}
What happens there is unrelated to Advanced-Search, but a CirrusSearch feature. It's not even a bug, but an intended feature.
See, the user typed kategorie:Údržba:Wikidata without quotation marks. I assume he wanted to find mentions of this category name on talk pages. But kategorie: is a CirrusSearch keyword. Prefixing a search with the name of a namespace tells CirrusSearch to search in this namespace, see https://www.mediawiki.org/wiki/Help:CirrusSearch#Prefix_and_namespace. This overrides the namespace selection below, no matter if done with the Advanced-Search form or without it.
Solution: The user needs to put quotation marks around "kategorie:Údržba:Wikidata", or use the Advanced-Search field that does this automatically.
I suggest to close this ticket as invalid.
This is effectively a duplicate of T223694: Misleading namespace input in Advanced Search when searching with a prefix as well as T210665: Auto-add namespaces if namespace input field fully matches their names.