Moved to MWG for better attention.
I had a similar problem with the 7.2.9 version, but only with Facebook. When the session time ended, the session exceeded page was not displayed, and the user were not able do anything else in the page, the page got frozen. When they close the browser or refresh the page, the new session page was shown correctly.
I opened a case with Platinum and they said its a problem with the Facebook ajax script, and because of that, no action could be taken on MWG. We had to instruct our users to press F5 everytime the session ended and Facebook freezes.
We still not tried it yet with 18.104.22.168, so I dont know if its the same problem, but it seems very similar.
If you are not doing Content Inspection with the SSL Scanner then the quota is activated by the initial CONNECT request and there is no way for the Web Gateway to interrupt the tunnel and inject the block page. The session will be exceeded but the tunnel will remain until it times out or is closed by the client or server.