Behavior of Service Templates is misleading
In https://docs.gitlab.com/ee/user/project/integrations/services_templates.html we read:
A GitLab administrator can add a service template that sets a default for each project. After a service template is enabled, it will be applied to new projects only and its details will be pre-filled on the project's Service page.
But actually, the service template is applied also on existing projects, which is not the intended behaviour.
Edited by Achilleas Pipinellis