I'm not sure I understand your issue. What problem does it cause?
The ending slash in Flarum is not configurable. The forum root page will always end with a slash, while all pages underneath end without a slash.
I believe that the config.php file must be set with the URL without the trailing slash. It's added by Flarum when building URLs.
When Flarum is installed in a subfolder, the ending slash for the homepage becomes visible in the address bar, whereas an installation without subfolder usually doesn't show the slash in the address bar, but that's because browsers hide the trailing slash on homepages. It's always there technically and is visible when sharing a URL to the current page.
The redirect from the Flarum folder name without trailing slash to the trailing slash is not done by Flarum, but probably by your webserver, or perhaps your WordPress htaccess file. It's not required to have such a redirect in place, but it's nice to have in case users type that URL manually.
Or do you mean the HTML canonical meta tag value is wrong on the trailing slash version of the homepage? I don't have a computer to check right now.
