Why are some environment variables missing when pre-receive is called by a merge request?

I have implemented a pre-receive hook that relies on some standard Git environment variables:


In recent GitLab versions (e.g. 14.0.5), when the pre-receive hook is called by a merge request, I noticed that these environment variables are now missing. Is this a bug or it is an intentional change? What is the motivation behind that?

Thanks in advance.