
This issue occurred from dotmailer/dotmailer-magento2-extension as detailed here: https://github.com/magento/magento2/issues/28244. It can be resolved by upgrading that module via
composer require dotmailer/dotmailer-magento2-extension:"4.5.1 as 4.4.0" php bin/magento setup:upgrade
Flush all the usual suspects, too: generated/*, caches, etc.

