Creating a merge request using a custom branch name loses all context of the issue

Summary

Using the extended dropdown popover to create a Merge Request from an issue fails to retain the context of the issue if you change the branch name.

Steps to reproduce

1. Go to any open issue
2. Click the arrow next to the "Create merge request" button
3. Change the pre-filled branch name to anything
4. Click the "Create merge request" button in the popover/to submit the form

Example Project

gitlab-org/gitlab

What is the current bug behavior?

The Merge Request creation page shows a "blank" Merge Request.

That is, there are no associated labels or milestones, the description does not include the ID of the referenced issue, and the title does not reference the issue title.

What is the expected correct behavior?

The resulting page should match what happens when you click the main "Create merge request" button (without opening the dropdown), or what happens when you click the "Create merge request" button in the dropdown when you don't change the branch name.

That is, the context should be associated with the new MR, including labels, milestones, updated description and title, etc.

Relevant logs and/or screenshots

Works	Disconnects the MR from the Issue

Output of checks

This bug happens on GitLab.com