Mp4 video not being served correctly to Safari

I have a website (https://mnkf.se) with an mp4 video no longer working in Safari from GitLab Pages. The video has been working for three months but stopped working in Safari today (at least I hope it was today) after a minor new deploy (did not change anything related to the video).

I can see in the developer tools that Safari requests the first bytes of the video but just says An error occurred trying to load the resource.. The console says Failed to load resource: Plug-in handled load. Has the way videos are hosted changed recently? It still works in Chrome.

The website is a simple static site which loads the video like this:

<video class="video" autoplay muted playsinline>
    <source src="assets/hero-720-3000.mp4" type="video/mp4">
</video>

.gitlab-ci.yml:

pages:
    stage: deploy
    script:
        - mkdir .public
        - cp -r * .public
        - mv .public public
    artifacts:
        paths:
            - public
    only:
        - master

The pipeline ran with gitlab-runner 13.6.0-rc1 (d83ac56c), which is a newer version than before.