Modification on merge commit message not used (default message remains)

Summary

When merging a merge request, the feature allowing to change the commit message isn't working properly.

Steps to reproduce

Use GitLab Enterprise Edition 10.8.3-ee gitlab-ee@3e7879a10cdfd5c54aaf025635d31a188289516f

  • Open a merge request

  • Assume that your policy is to create a merge commit each time you merge some merge request (no fast forward, rebase or squash)

  • Edit the commit message right before clicking the merge button of the merge request

  • Check the remove branch checkbox

  • Click the merge button

  • you can now check that the commit message available for the created merge commit isn't the message you typed in but the default message

What is the current bug behavior?

The default merge commit message remains after merge, even when modified before merging

What is the expected correct behavior?

The message typed in the textbox should be used as the commit message

Results of GitLab environment info

Expand for output related to GitLab environment info 
I can't run any command on the gitlab server installed.
Its version is GitLab Enterprise Edition 10.8.3-ee 3e7879a

Possible fixes

this feature used to work in 10.7.3 and isn't working in 10.8.3