Uncheck lightweight checkout instruction
Uncheck lightweight checkout instruction

For background, you can see https://github.com/jenkinsci/workflow-cps-plugin/blob/911e71375442d1cf0766b5dc4105babd72bd8265/src/main/resources/org/jenkinsci/plugins/workflow/cps/CpsScmFlowDefinition/help-lightweight.html which notes:
> build parameters will not be substituted into SCM configuration in this mode

Lots of people hit this at https://github.com/jenkinsci/ghprb-plugin/issues/507

I suspect the README instructions have other gaps - the instructions at https://stackoverflow.com/a/45121585/4200039 seem pretty good.
jenkinsci/ghprb-plugin,jenkinsci/ghprb-plugin