GitLab CI `rules:changes` always true on PR

[Potpourri]

PR creates from branch, but rules:changes always true for new branch, see more here: https://docs.gitlab.com/ee/ci/yaml/#using-onlychanges-without-pipelines-for-merge-requests

[Potpourri]

When new branch created by mirror sync, rules:changes work as expected.

[Potpourri]

rules:changes work only when:

• $CI_PIPELINE_SOURCE == 'push'
• $CI_PIPELINE_SOURCE == 'merge_request_event'

For $CI_PIPELINE_SOURCE == 'external_pull_request_event' it is always true. See issue: #37098

[Potpourri]

If use GitHub Required statuses for PR with rules:changes, then PR should have only one commit.
Since $CI_PIPELINE_SOURCE == 'push' calculates changes from previous commit, and not from branch base.