To overcome the issue of Missing self-referencing tag use this module and enable the setting from Store-> Catalog-> Catalog-> Seo-> Use Canonical Link Meta Tag For CMS Pages

The module is very simple to implement and also work with the Mageplaza Blogs extension.

To install the module run composer command : composer require mageride/mageride-canonical-cms

Run Commands :

php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy
php bin/magento cache:flush

After module enable flush the cache and check pages you can easily see the canonical link.