Pushing to a new branch runs all jobs when using only:changes

Summary

First off, apologies if this is not actually a ~bug but IMO the behavior is not what I expect.

The upshot of this issue is that builds on NEW branches for monorepos (with many jobs split up by only:changes to sub-dirs) are causing all jobs to trigger, making the build process much longer and computationally expensive than they should be.

To summarize the scenario:

• Take the following .gitlab-ci.yml file, along with 2 files ui.txt and packages.txt which I am using to trigger jobs based on changes:

stages:
    - build

.packages:
    only:
        changes:
            - packages.txt
    except:
        refs:
            - tags

build_packages:
    extends: .packages
    stage: build
    script:
        - echo 'build packages'

.ui:
    only:
        changes:
            - ui.txt
    except:
        refs:
            - tags

build_ui:
    extends: .ui
    stage: build
    script:
        - echo 'build ui'

If I commit a change to ui.txt and push directly to the primary branch (e.g. master or develop), then only build_ui runs, which is expected (since the only:changes condition is met).

The (Perceived) Bug

If I commit a change only to ui.txt and push to a NEW branch (e.g. chore/some-change), then BOTH build_ui and build_packages jobs run in the pipeline. Additionally, I have this same behavior if I create a new branch from the primary branch and make NO changes, and push remotely.

What I Expected

I expected the same behavior as when I push to the primary branch. Ideally, I'd like the changes to be computed based on the diff between the new branch and the branch it originated from.

My setup

• I have tried both the git clone and git fetch strategies, w/the same behavior.
• I have tried multiple permutations of various only:except:refs rules, with no luck just yet.

Sample Project

https://gitlab.com/tzellman/60105-jobs-test/

Pipeline that should have only run 1 of the jobs: https://gitlab.com/tzellman/60105-jobs-test/pipelines/55403567

Finally, thank you for all the hard work. This is not a major bug/issue, but the longer build times for new branches is causing some frustration.