Auto close ticket from merge request does not work anymore

Summary

Linked ticket does not get closed from merge request

Steps to reproduce

• Create an issue
• Create a linked merge request (and branch) from the issue page. MR gets created with linked issue
• Push changes and merge the MR
• MR gets closed but the ticket remains open

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behaviour, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug has been fixed in a more recent version)

What is the current bug behavior?

Ticket does not get closed

What is the expected correct behavior?

Ticket should get closed

Relevant logs and/or screenshots

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

Output of checks

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Results of GitLab environment info

Expand for output related to GitLab environment info

(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)

System information
System:		Ubuntu 16.04
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:	11.9.4
Revision:	4733950
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		xxxxxxx
HTTP Clone URL:	xxxxx
SSH Clone URL:	xxxxx
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: saml

GitLab Shell
Version:	8.7.1
Repository storage paths:
- default: 	/data/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

(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)