PHP page not loading; Wordpress taking over with 404

PHP page not loading; Wordpress taking over with 404

I have an issue where when I try to load a certain .php page in my
browser, on a domain with Wordpress installed, Wordpress is throwing a 404
instead of the page itself.
The problem page is:
http://healthysurprise.com/special/sign-up?product=healthy
You can see the analog page here (just remove '/special'):
http://healthysurprise.com/sign-up?product=healthy
The URL string is required for php to load the page dynamically. The page
is "sign-up.php" and is in the directory. What am I missing?
Notes:
- wordpress is installed
- index.php loads fine in both locations
- it seems to be doing this regardless of what the directory is
- there was a WP page template named 'sign-up' previously but I just
removed it
Here's my .htaccess

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
RewriteRule ^/([^-]*)$ /$1.php
Options +FollowSymLinks -MultiViews
# Turn mod_rewrite on
RewriteEngine On
RewriteBase /
## hide .php extension
# To externally redirect /dir/foo.php to /dir/foo
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.php [NC]
RewriteRule ^ %1 [R,L,NC]
## To internally forward /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^ %{REQUEST_FILENAME}.php [L]
</IfModule>
# END WordPress