Why is this page using the homepage template and not the one it should?

The difference between the two is that the test site has pretty permalinks enabled and the live site doesn’t. If you look at the URL, the issue becomes apparent, this is not a valid URL/query string:


If you change the second ? to an ampersand, the page renders correctly.

Either enable pretty permalinks on the live site, or modify your code with add_query_arg to correctly append the additional query argument:

<input type="hidden" name="redirect_to" value="<?php echo add_query_arg( 'register', 'true' ); ?>" />