Antora fails to download UI bundle containing gzipped files

So I have a bundle that is basically created with zip -r ui-bundle.zip ui-bundle/ on a ui-bundle directory formerly generated from https://gitlab.com/antora/antora-ui-default and antora is failing to generate a site with contradictory results:

• When I point an antora playbook's ui:bundle:url to the ui-bundle directory, everything works as it should.
• When I point ui:bundle:url to the local ui-bundle.zip generated with the above command (and similarly with 7z utility), then site generation fails with FATAL (antora): default layout not found
• When I point ui:bundle:url to my remote ui-bundle.zip generated with the above command, then antora outputs no logs whatsoever and also fails to generate the site. With some local testing, I have ended up that https://gitlab.com/antora/antora/-/blob/main/packages/ui-loader/lib/load-ui.js#L184 is not executed. But I have no idea why, and could not find where that event is coming from.
• When I point ui:bundle:url to the https://gitlab.com/antora/antora-ui-default/-/jobs/artifacts/HEAD/raw/build/ui-bundle.zip?job=bundle-stable then it generates the site without issues

The bundle can be found in ui-bundle.zip and also in https://gitlab.postmarketos.org/postmarketOS/handbook/-/jobs/artifacts/HEAD/raw/ui-bundle.zip?job=deploy

I'm a bit lost with the whole thing, so any pointers would be helpful