When an action of type "Set build description" is used as a promotion step for a build, let's call it BuildJobABC, it doesn't set the build's description.
Technically speaking, the promotion step doesn't fail. However, it sets the description of the promotion itself instead of setting the description of the build.
For example, if BuildJobABC has a promotion named PromoABC which is configured with an action of type "Set build description", the result of running PromoABC would be seen in the file "%JENKINS_HOME%\jobs\BuildJobABC\promotions\PromoABC\builds\PromotionId\build.xml" in the field "/hudson.plugins.promoted__builds.Promotion/actions/hudson.plugins.descriptionsetter.DescriptionSetterAction/description".
The expected behavior is for the promotion step to set the description of the build. This should be visible in the file "%JENKINS_HOME%\jobs\BuildJobABC\builds\BuildId\build.xml"