Mention non-participant in a merge request don't send mail

Summary

If we add a comment in : "Merge Request">"Changes">[select a line to add a comment, or even in the discussion tab]

@mention someone don't send email to the mentioned person

Steps to reproduce

In GitLab-ee 11.8.1-ee.0

add a new user (we will call him "userBug"), and give him a mail address

In his user settings > notification > Global notification level, choose "on mention"

With a other user go to "Merge Request">"Changes" and mention "userBug" (in my case "userBug" never work in this commit, idk if it's working for someone that work on the commit)

Example Project

Not really possible to show like this

What is the current bug behavior?

The mentioned person don't received any mail

What is the expected correct behavior?

He should be notified

Relevant logs and/or screenshots

I don't have any logs or screenshots for this...

But I'm ready to send if you need, just ask me

Output of checks

This bug happens on GitLab.com too (I just try to make sure)

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Debian 9.8
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.5.3p105
Gem Version:    2.7.6
Bundler Version:1.16.6
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.18.1
Sidekiq Version:5.2.5
Go Version:     unknown
GitLab information
Version:        11.8.1-ee
Revision:       39d0b2e
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
DB Version:     9.6.11
URL:            https://git.imnotshowingyouthis.com
HTTP Clone URL: https://git.imnotshowingyouthis.com/some-group/some-project.git
SSH Clone URL:  git@git.imnotshowingyouthis.com:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers: bitbucket
GitLab Shell
Version:        8.4.4
Repository storage paths:
default:      /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks
Git:            /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 8.4.4 ? ... OK (8.4.4)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
5/4 ... yes
5/5 ... yes
5/6 ... yes
5/7 ... yes
5/8 ... yes
5/9 ... yes
5/10 ... yes
5/11 ... yes
5/12 ... yes
5/13 ... yes
5/14 ... yes
5/15 ... yes
5/16 ... yes
5/17 ... yes
5/18 ... yes
5/19 ... yes
5/20 ... yes
5/25 ... yes
5/26 ... yes
5/27 ... yes
5/28 ... yes
5/30 ... yes
5/31 ... yes
5/32 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 16
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes

¯\(ツ)/¯

Thank you !