But for Cyrillic it is case sensitivе

[AlexD]

The Elastic Search works correctly and is case insensitive for both Latin and Cyrillic. If we use the Elastic Search Essentials plugin, it is not so.
Search through the Elastic Search Essentials plugin works correctly only for the Latin alphabet. But for Cyrillic it is case sensitive. For example, if the first letter of the request is a lowercase one, the search results will show only words that begin with a lowercase letter, and the same words that begin with uppercase letters will not be found.

 

[Xon]

@AlexD What stemming analyser are you using?

 

[AlexD]

Russian

Since the search after pressing the button works properly, looking for words that begin with capital and small letters, then the problem is not due Stemming analyser.