Thank you for your fast reply. Actually, this was the place where I started. Then I read all the searching section and then googled the problem and read the messages here. Unfortunately, I didn't find a solution and this is why I posted this message with a picture, believing that a picture worth 1000 words.
The search is simple, nothing fancy, just tag with a word, no special characters.
But it seems not to work in Filtered decks, regardless it does in search box. Documentation as I understood says that both are the same.... I cannot understad what is the problem and hope someone smarter than me to help me with this.
"Filtered decks can not pull in cards that are suspended, buried, or already in a different filtered deck. And unless you are using the experimental scheduler, they can not pull in cards that are in (re)learning. For this reason, a search in the browser may reveal cards that don’t end up in the filtered deck."
The cards in your screenshot are in learning or buried.
The card is indeed in learning. I'll dig further in this.
This is what I did and the result is puzzling for me, because in the browser card exists, in filtered deck - not. The only reason that might explain this behaviour is the one Damien pointed out - it is in learning.