Bounces don't make it back to the contact

Your software
My Mautic version is:2.16.3
My PHP version is: 7.3.24
My Database type and version is: MySQL 5.7.30

Your problem
My problem is: Mautic is touching bounce emails that come in, but is not marking the failures for those contacts. As a result, subsequent mailings are including the same contacts, even though they failed before.

These errors are showing in the log:

[2020-12-10 20:14:22] mautic.CRITICAL: Uncaught PHP Exception InvalidArgumentException: “The string b:8:{f:9:“fbhedf”;b:5:{v:3;f:8:“fzbvy”;v:4;v:66;}f:8:“fzbvy”;v:66;f:7:“fgbg”;f:55:“8ae59a3a13a7e544108968”;f:7:“yfbe”;f:9:“459207”;f:0:“dubaafy”;b:4:{f:8:“fzbvy”;v:66;}} is not a serialized array” at C:\inetpub\wwwroot\Mautic\app\bundles\CoreBundle\Helper\ClickthroughHelper.php line 46 {“exception”:"[object] (InvalidArgumentException(code: 0): The string b:8:{f:9:“fbhedf”;b:5:{v:3;f:8:“fzbvy”;v:4;v:66;}f:8:“fzbvy”;v:66;f:7:“fgbg”;f:55:“8ae59a3a13a7e544108968”;f:7:“yfbe”;f:9:“459207”;f:0:“dubaafy”;b:4:{f:8:“fzbvy”;v:66;}} is not a serialized array at C:\inetpub\wwwroot\Mautic\app\bundles\CoreBundle\Helper\ClickthroughHelper.php:46)"}

[2020-12-10 20:57:07] mautic.WARNING: The reference for contact 9290 and IP address 43361 is already there. (Unique constraint)

[2020-12-10 20:58:19] mautic.NOTICE: PHP Notice - Undefined index: ct - in file C:\inetpub\wwwroot\Mautic\app\bundles\PageBundle\Controller\PublicController.php - at line 452 {“redirectId”:“a0c700a6da9c06ba6cc19686b”,“logger”:"[object] (Symfony\Bridge\Monolog\Logger: {})",“redirectModel”:"[object] (Mautic\PageBundle\Model\RedirectModel: {})",“redirect”:"[object] (Mautic\PageBundle\Entity\Redirect: Mautic\PageBundle\Entity\Redirect with ID #153)",“url”:“https://go.verifiedfirst.com/nonprofit-hot-cocoa-hour","query”:}

Steps I have tried to fix the problem: Reviewed the monitored inbox settings, and they appear to be right. Also tried marking items in that folder as unread