Why does phpmailer "beat" some cyrillic characters?

Question

PHP

I seem to be doing everything right.
1) I specify the encoding when creating an instance $mail->CharSet = "UTF-8";
2) The encoding of the data and the script that creates the object and sends it is also UTF-8
The letter comes with the following fragments:
Update:
We deal with the author of the library here: PHPmailer cyrillic charset trouble

Answer 1

Possible multi-bit characters
To convert to utf-8

$value = mb_detect_encoding($value, mb_detect_order(), true) === 'UTF-8' ? $value : mb_convert_encoding($value, 'UTF-8');

encodings:

Answer 2

Do not pay attention to the content in the letter - it escaped) for the test