I found this from the WP 5.5 release notes, perhaps it is somehow related to this issue?
External Library updates in WordPress 5.5: call for testing
“PHPMailer (please test)
Because of conflicting PHP support policies (support for PHP < 5.5 in was dropped in newer versions of the PHPMailer library), WordPress Core has been stuck on the 5.x version of PHPMailer for quite some time, even though a newer, 6.x version exists. However, now that WordPress only officially supports PHP back to version 5.6.20, PHPMailer can finally be updated.
This is a major update to the library. Please help by testing this update to ensure any edge cases are discovered.
Plugins that appear to be utilizing the library will soon receive an email to test their code. Below is what you need to know about the changes as they relate to WordPress Core.
Note: this is not an exhaustive list. For a more complete list of changes, please consult the Upgrading from PHPMailer 5.2 to 6.0 guide.
New file locations
To make updating the library easier, the PHPMailer files have been moved into the wp-includes/PHPMailer directory. The old files will remain for backwards compatibility and load the new corresponding files, but loading them will now throw a _deprecated_file() warning.
wp-includes/class-phpmailer.php is replaced by wp-includes/PHPMailer/PHPMailer.php.
wp-includes/class-smtp.php is replaced by wp-includes/PHPMailer/SMTP.php.
Additionally, the phpmailerException class is now separated into its own file, wp-includes/PHPMailer/Exception.php (previously, it was included at the bottom of the class-phpmailer.php file). When loading the PHPMailer library, the Exception.php file is required, and should also be loaded. If the old class-phpmailer.php file is loaded, the exception class will be loaded automatically to minimize failures.
New PHPMailer namespace
In past versions, the PHPMailer class existed within the global space. However, moving forward, the PHPMailer library is under the new PHPMailer\PHPMailer namespace. Below is an example of the old usage, and an example of the new way to utilize the library.
Old method
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<?php
require_once ABSPATH . WPINC . ‘/class-phpmailer.php;
$phpmailer_instance = new PHPMailer();
New methods
Both examples will produce identical results.
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<?php
use PHPMailer\PHPMailer\PHPMailer;
require_once ABSPATH . WPINC . ‘/PHPMailer/PHPMailer.php;
require_once ABSPATH . WPINC . ‘/PHPMailer/Exception.php;
$phpmailer_instance = new PHPMailer();
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<?php
require_once ABSPATH . WPINC . ‘/PHPMailer/PHPMailer.php;
require_once ABSPATH . WPINC . ‘/PHPMailer/Exception.php;
$phpmailer_instance = new PHPMailer\PHPMailer\PHPMailer();
Similar changes must also be made when utilizing the PHPMailer Exception class.
The old classes have been registered as aliases of the new, properly namespaced classes using class_alias() in PHP (this is true for the PHPMailer, phpmailerException and SMTP classes). This means that a plugin containing code similar to the “old method” will continue to work seamlessly until it can be updated (with the exception of the _deprecated_file() notice).
Note: the aliases will only exist if the old, deprecated files are included. A plugin relying on Core to load the PHPMailer library will not have an alias present as the new files are now loaded instead.
Deprecated elements have been removed
Another big change in the 6.x update was the removal of all deprecated properties and methods. A full list of removed elements can be found in the upgrade guide.
Plugins using PHPMailer
All plugins that appear to be utilizing PHPMailer will soon be receiving an email notification to test their code and make any necessary adjustments.
For more information, check out the full list of changes included with this update, the PHPMailer 6.0 upgrade guide, or the related Trac tickets (#41750, #50379, #50380).”
