Steps to reproduce the issue

• install 4.0.0
• install last version of patchtester
• try to install joomla/joomla-cms#12132
• try to send a mail with the send mail buton
• it fails
• look at https://github.com/mbabker/joomla-cms/blob/PHPMailer-6.0/libraries/vendor/phpmailer/phpmailer/src/
• and comepare it with your dir
• you see the PHPMailer.php is missing
• have a look at the PR
• this file gots renamed in the PR but not on your site

Expected result

Patch successfull applyed

Actual result

Patch not successfull applyed

System information (as much as possible)

• J4.0.0
• last patchtester