Why is Apache Permanent Redirect removing the slash between the domain and the path?

I got it.

The issue did not lay with the rewriting at all, it was the SSLRequireSSL directive under my Directory definition that was causing the problem.

I simply removed this directive, refreshed the cache in all of my browsers, and the site then continued to work correctly. This was discovered through the process of elimination.

The documentation notes:

This directive forbids access unless HTTP over SSL (i.e. HTTPS) is enabled for the current connection. This is very handy inside the SSL-enabled virtual host or directories for defending against configuration errors that expose stuff that should be protected. When this directive is present all requests are denied which are not using SSL.

The emphasis is my own. SSLRequireSSL may have Apache only return a 403 or 404 if HTTP over SSL is not enabled, interfering with the Redirect rule. A rewrite rule such as the one in this answer on Server Fault may be a better alternative depending on your use case:

RewriteEngine On
RewriteCond %{SERVER_PORT} !443
RewriteRule ^(/(.*))?$ https://%{HTTP_HOST}/$1 [R=301,L]

My issue was related to browser caching.

I tried it in a different browser and it worked and then tried again in a private session in the first browser and it also worked.