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Your English is fine. The behaviour of LR is odd though. One would think that "Any Searchable Field" would include Caption but it does not. Thank goodness we can make a custom set.
Yes it's odd behavior. But if you know the data is in the Caption you can search Captions and it will find it. Since you can search Caption one would think it's a "searchable field" but LR doesn't think so.
Sounds like a bug. There's a link at the top of the page for reporting bugs to Adobe.