"Merge" button appears instead of "Start merge train" when manual jobs aren't triggered.

Summary

Steps to reproduce

1. Create a project, in the Settings > Merge Requests, Enable Enable merged results pipelines and Enable merge trains

2. Create a file .gitlab-ci.yml with the job:

job1:
    rules:
      - if: $CI_MERGE_REQUEST_ID
        when: manual
    script: echo "something"

3. Create a new branch, create a new file in that branch
4. Create an MR of the new branch to the "main" branch

Example Project

https://gitlab.com/gitlab-gold/asalii/train-manual-block

What is the current bug behavior?

The button that should be "Start merge train", is "Merge" instead

What is the expected correct behavior?

The button should be "Start merge train"

Possible fixes