Theme Customiser not working when WordPress on sub-directory

I installed wordpress in a subdirectory and used .htaccess to redirect the domain so my domain would be https://www.example.com as opposed to https://www.example.com/subdirectory/ or subdirectory.example.com. I did this following the steps outlined here. In doing this I switched the Site Address (URL) in Settings > General to https://www.example.com from subdirectory.example.com but left the WordPress Address (URL) as subdirectory.example.com.

The issue I’m having now is that when I try to customize my theme within WordPress (Appearance > Customize), I get a prompt to login. I enter the credentials correctly as always but it keeps asking me to login, even when the login was successful. At the top of the login form is the message “Your session has expired. Please log in to continue where you left off.”

When I changed the Site Address (URL) back to subdirectory.example.com, the Customizer works as normal.

The URL during the issue is: subdirectory.example.com/wp-admin/customize.php?return=%2Fwp-admin%2Foptions-general.php

How do I make it such that Appearance > Customize works with my desired domain configuration. Any assistance would be greatly appreciated.