The plugin redefines the standard CustomEvent function in an old-fashioned way, without testing if the function already exist:

https://github.com/jenkinsci/extended-choice-parameter-plugin/blob/7ee42bc81a898a4bb6cea053d270ca9347b2a4bb/src/main/webapp/js/jsoneditor.js#L85C1-L96

This implementation uses deprecated initCustomEvent, which doesn't properly handle the composed property , which is always false. This property is heavily used with shadow dom and web components, and should be configurable to true.