addGitLabMRComment does not work with Multi-branch pipeline

### Jenkins and plugins versions report

Environment

```text
Paste the output here
```

### What Operating System are you using (both controller, and any agents involved in the problem)?

Linux

### Reproduction steps

Add "addGitLabMRComment" to steps

post {
unstable {
echo "This block runs if the current status is marked unstable."
addGitLabMRComment comment: 'Unit tests failed'
}
}

### Expected Results

MR comment should be added to GitLab.

### Actual Results

MR comment is not added to GitLab.

### Anything else?

_No response_