Quick action for /close #x disappears from merge request description

Summary

When adding a quick action to a merge request description (such as `/close #x'), the quick action disappears. Editing the merge request before merging allows the quick action to show up.

Steps to reproduce

Tests in GitLab EE version 11.6

1. Create a new project with an empty README.md
2. Create an issue
3. Create a branch called 'test'
4. Edit README.md in the test branch
5. Commit README.md to test branch
6. Create a merge request from 'test' to 'master', putting /close #x in the description (where x is your issue number from step 2)
7. Click 'Submit merge request'
8. Verify on the merge request that issue x is NOT listed on the issues to be closed
9. Click 'Edit' to edit the merge request
10. Verify that the /close #x action is not present
11. Re-add the /close #x quick action
12. Click 'Save'
13. Verify that the merge request now appropriately lists that it will close issue x

If we merge after step 13, the merge will close issue x. If instead we merge after step 8, the merge will not close the issue.

What is the current bug behavior?

Close quick actions are lost when creating a merge request and only show up after editing a merge request

What is the expected correct behavior?

Close quick actions should be appropriately added to the merge request from the merge request description when it is created without needing to edit the merge request description.