The Gradle build relies on annotated tags to compute the version. Creating a tag on GitHub when drafting a new release does not create an annotated tag, surprisingly. A GitHub tag does have a nice "verified" qualifier, which can also be achieved by pushing a signed tag:
git tag -s -a <version> -m ':bookmark: <version>'
To check that Gradle extracts the version properly do:
./gradlew version
Push the tag to GitHub and draft a release manually based on this tag:
git push origin <version>
Publishing to the plugin portal still has to be done manually:
./gradlew publishPlugins