I tried to use the git parameter plugin in a pipeline script to fill an input (Target Branch to merge to in another repository).

According to https://jenkins.io/doc/pipeline/steps/pipeline-input-step/ GitParameters should work for inputs, 

input(
  id: 'Proceed1', message: 'Specifiy QA Candiate Branch', 
  parameters: [
    [$class: 'GitParameterDefinition', 
     name: 'candidateBranch', 
     type: 'PT_BRANCH', 
     ]
  ]
)

This gets rendered but whatever I do the branch select stays empty (When refreshing I can see a brief spinner with "Retrieving..."). I also tried to capture related logs but couldn't get anything (From looking at the code only exceptional cases are logged).

I also tried as documented in https://jenkins.io/doc/pipeline/steps/pipeline-input-step/

type: 'Branch' 

Just to validate that it works in general I added the parameter to the job itself and if I trigger a parameterized build the select is properly filled.