I configured a multibranch pipeline to check out via git, discover branches, discover tags and the added clone behaviour to fetch tags.

However I can't use the when condition with tags as outlined here (https://jenkins.io/blog/2018/05/16/pipelines-with-git-tags/). According to the docs (https://jenkins.io/doc/book/pipeline/syntax/#when) when depends on the variable TAG_NAME which I assume should be set by Jenkins. But both  params.TAG_NAME, env.TAG_NAME are null.

Thus my when condition 'when { tag "" }' never triggers.