Register_Sidebar overwriting itself and doesn’t exist in global $wp_registered_sidebars;

The solution was simple and made me feel tremendously stupid.
90% of the issues I could find remotely similar to my issue were all issues relating to the casing. Throughout this whole process I overlooked the “_HOMEPAGE” and “_RIGHT_SIDEBAR” in my IDs.

Those should be lowercase.

id – Sidebar id – Must be all in lowercase, with no spaces (default is a numeric auto-incremented ID). If you do not set the id argument value, you will get E_USER_NOTICE messages in debug mode, starting with version 4.2.

https://codex.wordpress.org/Function_Reference/register_sidebar#Parameters

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