"Merge" button appearing on MRs even though Merge Trains are enabled

Summary

On the gitlab-com/www-gitlab-com project, Merge Trains are enabled. However, when creating a new merge request from a branch, the green "Merge" button initially appears for a period of time. This allows users to bypass the merge train and MR pipelines, and merge directly to master.

Steps to reproduce

• On www-gitlab-com repo.
• Make a branch and push it.
• On the GitLab project, go the Repository - Files page.
• At the top, next to the "You pushed to just now", message, click the "Create Merge Request" button
• On the New Merge Request screen, assign the merge request to yourself, and click the "Submit Merge Request" button

Example Project

• This is a MR on which the bug was reproduced: gitlab-com/www-gitlab-com!46836 (merged)
• Video of the bug (sorry for the low quality, had to compress to upload): bypassing-merge-train
• Screenshot:

What is the current bug behavior?

The green "Merge" button appears, and can be clicked to merge immediately to master.

What is the expected correct behavior?

The "merge" widget should show the blue "Merge Train" button/dropdown.

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's tough to read otherwise.)

Output of checks

This bug happens on GitLab.com