Windows Search has an ancient bug that Microsoft cannot fix in any way since 2009.
Windows Search doesn't work correctly with the files with Cyrillic text (perhaps with another non-English text) encoded in UTF-8 (for example, .txt).
Files "Notes.md" in TheBrain encoded in UTF-8.
This is the reason why the notes with Cyrillic text are not displayed in the search.
If you change UTF-8 to UTF-8 with signature (UTF-8 with BOM) everything works fine !!!
TheBrain 11 works perfectly with files "Notes.md" encoded in UTF-8 with signature (UTF-8 with BOM) !!!
Please, change UTF-8 to UTF-8 with signature (UTF-8 with BOM) for the files "Notes.md" in the next build of TheBrain 11 to default for search the notes with Cyrillic text or add the ability to change the encoding for notes !!!
Because Microsoft is unlikely to fix it, judging by the posts in the Microsoft community ...
Important: this search bug applies to all versions of TheBrain, that use Windows Search.