Sending messages in Cyrillic and PHP 8

I'm running into a problems when sending messages in Cyrillic with Contact form (as well as with Webform) on a Backdrop CMS site:

The message is received from the sender's email and the Cyrillic text reads normally when reading the message in Thunderbird / Windows 10 email client.

However, when receiving the message by an email client on an Android smartphone (the e-mail client is K9 Mail), the Cyrillic text is replaced with unintelligible characters.

Here is an example of the K9 email client for Android:

�������� Test (abc@email.com) �������������� ���������������� �� ������
������������������ ���� �������������� https://example.com/contact.

�������������� ���������� ���� �������� Test: 12345678

�������� 3 Test 1 PHP 8.3

When reading the same message through Roundcube web-based e-mail that is part of CPanel's web server applications, Cyrillic in messages is also unintelligible:

Тест Test (abc@email.com) изпрати съобщение чрез
страницата за контакти https://example.com/contact.

Телефонен номер на Тест Test: 12345678

Тест 3 Test 1 PHP 8.3

In both examples, it can be seen that the problem affects only the part of the text that is in Cyrillic, but the Latin is read normally. The problem affects both the Subject and the body of the message.

This problem only occurs when the PHP version of the web server is changed to PHP 8 (the problem occurs equally with PHP 8.0, PH 8.1, PHP 8.2).

If the web server is running PHP 7.4 there is no problem and Cyrillic messages sent through the message form are read normally.

For comparison, I also sent a message via a Webform module message form:

When running the web server with PHP 7.4, there are no problems again.

Changing the PHP version to 8.0 results in the following message sent via Webform

�������������� ��� 11
�������������� ���� ������������, �������������� 19 , 2023 - 5:39pm

���������� ���� ����������������������:

����������: Test
������, ��������������: Test
��������������: 12345
Email: abc@email.com
������������ ���� ��������������: ���������������� � ���� ��������������:

Test web server in this test is running PHP 8.0. If the web server in is running PHP 7.4 there are no problems with Cyrillic content in messages.

The output shows for me that it seems the problem is valid for both the contact form module and the messages sent through the webform module.

This problem is preventing me from upgrading the web servers of the wesites that I administer (they are all managed by cPanel) to PHP 8, and the lifetime of PHP 7 is decreasing alarmingly.

I would greatly appreciate any advice on how to try to solve this problem. Maybe there are some message code table settings that can be edited in the Backdrop settings.