One thing is that even though Xcode SCM is configured with HTTPS and the repo has only HTTPS remote configured when setting up Xcode the the authorisation flow flashes this redirect URL:
client_redirect_path=/ci/api/teams/REDACTED/internal-setup-flow/scm-providers&team_id=REDACTED&scm_provider_id=REDACTED&client_redirect_base_uri=https://appstoreconnect.apple.com?***git_clone_url=ssh://git[@]gitlab._REDACTED_/_REDACTED_.git***&provider=gitlab-hosted&redirectAfterSuccessUri=https://appstoreconnect.apple.com/teams/_REDACTED_/ci-setup/success&code=_REDACTED_&state=_REDACTED_
Why is Xcode Cloud attempting to clone via SSH?
git_clone_url=ssh://git[@]gitlab.REDACTED/REDACTED.git
