tag barfs too, but you have to open the "Advanced" options to get it to do so.

Got it - PR up at https://github.com/jenkinsci/pipeline-model-definition-plugin/pull/261 fixing that and making a few other changes to when condition directive generation.

Code changed in jenkins
User: Andrew Bayer
Path:
pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/generator/WhenDirective.java
pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/BuildingTagConditional.java
pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ChangeRequestConditional.java
pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/TagConditional.java
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/generator/WhenDirective/config.jelly
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ChangeRequestConditional/config.jelly
http://jenkins-ci.org/commit/pipeline-model-definition-plugin/eac90be72124d7ef32d957af898a5eef913f478b
Log:
[FIXED JENKINS-50682] Actually pass descriptor to when condition forms

Also add display name for a few when conditions, move all of
ChangeRequestConditional's fields into advanced, since they're
optional and all, and smack things around a bit internally to not
display when directive parameters when they're not required and
they've got an empty string value.

Merged, releasing today in Declarative 1.2.9.

Bulk closing resolved issues.