Only show levels on levels page which the original referral page belonged to.

pmpro-restrict-levels.php

/*
Only show levels on level page which the original content belonged to. For example if a site has 3 levels: Bronze, Silver, Gold and
a post is restricted to Silver and Gold, clicking on the "Register" button will lead them to the levels page showing only Silver and Gold levels.

To use: employ the shortcode [my_level_link] under Memberships >> Advanced >> Message for Logged-out Users (or Message for Logged-in Non-members)

Ex:

This content is for !!levels!! members only.<br /><a href="http://example.com/wp-login.php">Log In</a> <a href="[my_level_link]">Register</a>
*/

function my_level_link()
{
	global $post;
	$hasAccess = pmpro_has_membership_access($post->ID, null, true);
	
	$access = site_url()."/membership-account/membership-levels/?";
	
	foreach($hasAccess[1] as $key => $value)
	{
		$access = $access."levels[]=".$value."&";
	}
	
	return $access;
}

add_shortcode('my_level_link', 'my_level_link');

function my_pmpro_levels_array($levels)
{
	$allowed_levels = $_REQUEST['levels'];
	
	$newlevels = array();
 
	foreach($levels as $level)
	{
		if(in_array($level->id, $allowed_levels))
			$newlevels[] = $level;
	}
 
	return $newlevels;
}
add_filter("pmpro_levels_array", "my_pmpro_levels_array");

Forked to add a check if going to levels page from a direct link no on a protected page https://gist.github.com/kimwhite/2ff0eff060badbbebbddfcf20cc74269