Two-step merge is not merged automatically

Summary

I tried to merge a branch into master, then production. The second step was not performed automatically

Steps to reproduce

1. Branch "cool-feature" with all CI builds passing
2. Create merge request gitlab-ce!1 to merge "cool-feature" into "master"

• Automatically merge when build succeeds is ticked
• Remove source branch

3. While CI build for 2. is still running, create merge request gitlab-ce!2 to merge "master" into "production"
4. Accept merge request gitlab-ce!2
5. Build succeeds

Expected behavior

6. Branch "cool-feature" is removed
7. Commits from branch "cool-feature" should be in "master"
8. Commits from both "master" and "cool-feature" should be in "production"

Actual behaviour

6. Branch "cool-feature" is removed
7. Commits from branch "cool-feature" are in "master"
8. Merge request gitlab-ce!2 is now hanging in mid-air, nothing happens (see screenshot)

Relevant logs and/or screenshots

Output of checks

Results of GitLab application Check

I decline to paste the output of sudo gitlab-rake gitlab:check SANITIZE=true because it outputs the LDAP DN for all users. I think the "sanitize" option is broken.

Everything else is fine, though.

Results of GitLab environment info

System information
System:		RedHatEnterpriseServer 7.2
Current User:	git
Using RVM:	no
Ruby Version:	2.3.1p112
Gem Version:	2.6.6
Bundler Version:2.3.0
Rake Version:	10.5.0
Sidekiq Version:4.1.4

GitLab information
Version:	8.11.2
Revision:	e618ce2
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		http://gitlab.hq.stgt.etes.de
HTTP Clone URL:	http://gitlab.hq.stgt.etes.de/some-group/some-project.git
SSH Clone URL:	git@gitlab.hq.stgt.etes.de:some-group/some-project.git
Using LDAP:	yes
Using Omniauth:	no

GitLab Shell
Version:	3.4.0
Repository storage paths:
- default: 	/storage-sata/usr/share/git/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:		/opt/gitlab/embedded/bin/git