problem with form data

From day 1 CodeIgniter has always used Controller/Method with no / on the end for the URL of Form Data.

Yes, and the issue has nothing to with CodeIgniter4.

POST appointment/ → Apache mod_rewrite redirects it to appointment
GET appointment → $this->request->getPost() is empty.

# Redirect Trailing Slashes...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} (.+)/$
RewriteRule ^ %1 [L,R=301]

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