"Include merge request description" or any custom merge commit message not working with Merge Trains enabled

Summary

In a configuration with "Enable merge trains and pipelines for merged results" the option "Include merge request description" does not add the description to the commit message.

Steps to reproduce

Setup a project with "Enable merge trains and pipelines for merged results", open a Merge Request and set "Include merge request description". See commit history.

Example Project

https://gitlab.com/till.steinbach/new-project/

What is the current bug behavior?

Commit message does not contain merge request description

What is the expected correct behavior?

Commit message should contain merge request description

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com