Background
Ask your system admin to check the database settings (at least the Postgres DB is affected). For the locale configuration recommended by Atlassian (documentation) the text search may not work correctly for specific symbols.
Solution
The system administrator should change the DB parameters as follows (for Postgres and Russian language):
From: ENCODING 'UNICODE' LC_COLLATE 'C' LC_CTYPE 'С'
To: ENCODING 'UNICODE' LC_COLLATE 'C' LC_CTYPE 'ru_RU.UTF-8'