CI trigger is not right

I believe something is wrong with this CI rule :

  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event" || $CI_COMMIT_REF_NAME =~ /master/

It triggers a pipeline even for small things like changing the target branch of the MR. And also there is https://gitlab.orfeo-toolbox.org/nicolasnn/pyotb/-/merge_requests/56#note_104223

Edit: still an issue : the '||' or is not working as expected, and with several "if" statements it's not right either