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I created small text which is bellow. And I will give some explanation after code.

\documentclass{amsart}
\def\Defined{}
\scrollmode
\usepackage{cmap}
\usepackage[T2A,T2B]{fontenc}
\usepackage[cp1251]{inputenc}
\UseRawInputEncoding
\usepackage[english,russian]{babel}
\selectlanguage{russian ...

I use package Babel to compile in LaTeX Russian text. After recent changes, package Babel converts letter ё into letter ч. Even this letter is very rare, text of pdf file looks very strange.

Thank you