Thanks for sharing your sandbox! I see you are using an older properties panel version.
but using the new API
Please consider updating the version: https://github.com/bpmn-io/bpmn-js-properties-panel/blob/master/CHANGELOG.md#100-alpha5
And also adding the necessary peer-dependency: https://github.com/bpmn-io/properties-panel
I adjusted your sandbox to make it work: bpmn-modeler (forked) - CodeSandbox