To make sure I'm following, are you finding that the grades are being passed from the tool but not showing up in Moodle? If so, FWIW we experienced that and after much testing and checking of logs we found out it was how https requests turn into http requests inside our system. This was the solution that worked for us (we were on 2.6 at the time):
Modify Moodle to switch out http for https when it's calculating the signature, knowing that whatever was sent to it from the external world was indeed sent to an https:// address.
You would have to carry this change forward as you upgraded Moodle, of course, so be aware of that.
The method you would have to change is "get_normalized_http_url()" in mod/lti/OAuth.php around line 403, possibly by inserting the code:
if ($scheme == "http") {
$scheme = "https";
}
right before the "return "$scheme://$
host$path" line at the end of the method