I tried to create a git tag with semantic versioning 0.3.0 and executed it locally with the --event=tag flag. Now my docker step gets executed, but it is just creating a latest tag with auto_tag: true in the repository. Only when I explicitly specify my version in tags and omit auto_tag the version is used. I also tried using tags: ${DRONE_TAG} which yields Error parsing reference: "********:" is not a valid repository/tag: invalid reference format.
I think there is an issue where Gogs does not work with autotag due to lack of commit sha being provided in the Gogs payload [1]. This may also impact Gitea since it is a fork of Gogs, although I think the Gitea folks patched this recently.
But otherwise, you will find that we use auto_tag for Drone and all Drone plugins. Looking at some real-world configurations may be useful [2][3]