HTTPS redirect not working on a specific endpoint

I am using the following in my .htaccess file to force HTTPS

RewriteEngine On
RewriteCond %{HTTPS} off [OR]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^(?:www\.)?(.+)$ [NC]
RewriteRule ^ https://www.%1%{REQUEST_URI} [L,NE,R=301]

This works perfectly for all pages except one page - /workbook

When visiting you are redirected to /index.php rather than added the correct https prefix like it does on all other pages.

This only happens when http and no www are used. If http and www are used together it does works. No other page on the website is behaving like this, just /workbook.

Additionally if a change the URL to something else ie /workbooktest it redirects correctly, but changing the URL isn’t possible as it’s linked in a physical book.

What could be different about this particularly URL that Kirby doesn’t like?

If I enter that URL in my browser, it correctly redirects to https://www./ Maybe some browser caching on your side?