Once the user logs in to Discourse again, the e-mail address will be re-synced. As the help text for sso overrides email says:
Overrides local email with external site email from SSO payload on every login
(Emphasis added)
If you want to see the change immediately reflected in Discourse, you’ll need to have your Drupal site make an API call into Discourse to update the user’s details (which, now I think about it, might not work with sso overrides email… you’ll need to test that). This is necessary because Discourse isn’t constantly pinging the SSO provider saying “hey, what’s the e-mail for this user now? How about now? Aaaaaand… now?”, so the only time Discourse gets told that the e-mail address has changed (absent the sort of API call I suggested above) is when the user logs in and the SSO provider gives Discourse the bundle of user data to use.