Currently the git plugin doesn't seem to support tags in multi-branch builds. It looks like this is because the ref prefixes are hard-coded. See: https://github.com/jenkinsci/git-plugin/blob/master/src/main/java/jenkins/plugins/git/AbstractGitSCMSource.java#L417
This is similar to
JENKINS-34395 but for the git plugin. I'm creating a separate issue because the causes and the fixes appear to be unrelated.