How to tell if a pipeline with `CI_PIPELINE_SOURCE=merge_request_event` is for an MR creation or push to an open MR?

GitLab CI/CD
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1

Problem to solve

We know that a CI_PIPELINE_SOURCE=merge_request_event pipeline is created in two scenarios (at least). One when an MR is created and another when a push commit is made to an open MR.
We need a solution to tell if the merge_request_event pipeline belongs to creating an MR or to a push to an existing open MR? Looking at the env vars in the jobs, there is no obvious one that can provide this information.

I was hoping these variables help, but they are identical in both pipelines:

CI_PIPELINE_SOURCE == merge_request_event
CI_COMMIT_BRANCH == null
CI_OPEN_MERGE_REQUESTS != null
CI_COMMIT_BEFORE_SHA = 0000000000000000000000000000000000000000 
CI_MERGE_REQUEST_ID != null

Version:      17.0.2
Git revision: 8ad74cf9
Git branch:   17-0-stable
GO version:   go1.21.9
Built:        2024-07-25T17:34:34+0000
OS/Arch:      linux/amd64