Merge request approvers in list sometimes doesnt show correct number
### Summary
(Summarize the bug encountered concisely)
In my test instance, I created multiple merge requests that require 2 approvers as Administrator and got this as a result.

Where only the one 3 days old shows the correct number.
I tried to reproduce this with my dummy account on my test instance but
never got that to work.

Now this problem doesn't happen all the time.
I tried several steps to try and reproduce this.
### What is the current *bug* behavior?
1. Set approvers number in the merge request, created a merge request.
2. Set approvers number in the settings, created a merge request.
3. Set selected approvers in the settings, created a merge request.
Now depending on the project this either shows the correct number at `/merge_requests` while sometimes it just shows `0 of 0`
### What is the expected *correct* behavior?
It should always show the correct number of approvers required and already approved.
#### Results of GitLab environment info
<details>
<summary>Expand for output related to GitLab environment info</summary>
<pre>
root@rz-gltest:/# sudo gitlab-rake gitlab:env:info
System information
System: Ubuntu 16.04
Proxy: no
Current User: git
Using RVM: no
Ruby Version: 2.6.3p62
Gem Version: 2.7.9
Bundler Version:1.17.3
Rake Version: 12.3.2
Redis Version: 3.2.12
Git Version: 2.21.0
Sidekiq Version:5.2.7
Go Version: go1.6.2 linux/amd64
GitLab information
Version: 12.0.2-ee
Revision: ef76b54fc1e
Directory: /opt/gitlab/embedded/service/gitlab-rails
DB Adapter: PostgreSQL
DB Version: 10.7
URL: http://178.128.44.26
HTTP Clone URL: http://178.128.44.26/some-group/some-project.git
SSH Clone URL: git@178.128.44.26:some-group/some-project.git
Elasticsearch: yes
Geo: no
Using LDAP: no
Using Omniauth: yes
Omniauth Providers: gitlab
GitLab Shell
Version: 9.3.0
Repository storage paths:
- default: /var/opt/gitlab/git-data/repositories
GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell
Git: /opt/gitlab/embedded/bin/git
</pre>
</details>
#### Results of GitLab application Check
<details>
<summary>Expand for output related to the GitLab application check</summary>
<pre>
root@rz-gltest:/# sudo gitlab-rake gitlab:check SANITIZE=true
Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 9.3.0 ? ... OK (9.3.0)
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: ...
1/1 ... yes
1/2 ... yes
2/3 ... yes
1/4 ... yes
1/5 ... yes
4/6 ... yes
13/7 ... yes
5/8 ... yes
1/9 ... yes
1/10 ... yes
4/11 ... yes
1/12 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.3)
Git version >= 2.21.0 ? ... yes (2.21.0)
Git user has default SSH configuration? ... yes
Active users: ... 3
Elasticsearch version 5.6 - 6.x? ... yes (6.6.0)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished
</pre>
</details>
### Possible fixes
Zendesk: https://gitlab.zendesk.com/agent/tickets/124778 (internal)
issue