Generated composer.json seems to ignore my desired core version

Hmm, reading more into it, this is exactly what OPT_IN_TEST_NEXT_MINOR is for.

https://git.drupalcode.org/project/gitlab_templates/-/blob/main/includes...

The templates set the version of Drupal expected for the variant being run. If you want to keep the one set in the module's composer.json you need to add a variable to the composer job as shown in here https://project.pages.drupalcode.org/gitlab_templates/jobs/composer/#pac...

But yeah, if you want to test against the HEAD, the NEXT_MINOR variant is exactly that at the moment.

Thanks for confirming!