When upgrading ZenCart it is easy to miss certain files during the merge process of the upgrade. There are numerous directories that are easy to look over but two directories to note include the includes/modules/YOUR_TEMPLATE/ and includes/languages/english/YOUR_TEMPLATE directories. It is easy to overlook files that may have been modified in either of these locations. The HOME_PAGE_TITLE language tag would more than likely be from the includes/languages/english/YOUR_TEMPLATE/meta_tags.php.
During the upgrade process the includes/languages/english/meta_tags.php file was probably replaced but if there have ever been any modifications based on template to the site then you need to look into your templates directory located in the languages folder.
To resolve merge the changes from the upgraded meta_tags.php file to the meta_tags.php file located in the directory with your templtaes name located in the languages directory. The HOME_PAGE_TITLE text is showing because it is a variable that does not have a value assigned to it. ZenCart allows you to easily switch between languages by adding custom text to each languages file structure located underneath the includes/languages directory.
File to Merge:
Hopefully this will help you track down the issue with your ZenCart store META tags.