Group Email Notifications use the wrong address for comment emails

Summary

(NOTE: title is a guess, root cause still unclear).

Group-level email routing does not appear to be working correctly. When setting an email address for a GitLab group that is not the Global Email notifications are still being received on global email address.

I've recently enabled group email notifications to distinguish between my GitLab employee projects and my personal GitLab projects. I have now configured my global email to my personal account and setup all GitLab company groups to my *@gitlab.com address. I am still receiving a large number of emails to my global email, however.

Happy to provide more details but the pattern by which I'm receiving emails is not immediately clear. My best guess is that these are issues I've subscribed to previously which are not applying my new group-level routing preferences.

Steps to reproduce

Untested theoretical steps

1. Subscribe to an issue's notifications
2. Change primary email to different email
3. Change group containing issue to previous email address
4. Receive notification on issue and observe routing

Example Project

What is the current bug behavior?

Notifications for non-global-email group are still being sent to global email address

What is the expected correct behavior?

All notifications for a group with a configured group-level email should be sent only to that email address

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's tough to read otherwise.)

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:env:info)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)