The git additional behavior step this plugin introduces help text says:

If the revision has more than one tag associated with it, only the most recent tag will be taken into account, unless the refspec contains "refs/tags/" — i.e. builds are only triggered when certain tag names or patterns are matched — in which case the exact tag name that triggered the build will be used, even if it's not the most recent tag for this commit.

So if a tag has multiple commits, but has a refspec that “contains” refs/tags/,
if the build is triggered with a specific tag (git push hook) then that tag is being used.

However, this is not the case. It seems to use the first tag after alphabetically sorting them.

I have a commit with five tags which I pushed one after the other individually to test. Instead of the last one I pushed (nr 5) it built the first tag I had pushed.

Pushing another test tag that would be alpha-numerically-sorted before the other tag it did build the pushed tag.