This needs to be changed in both pipeline-input-step and blueocean, since there's more-or-less duplicate code in those places.
Persist accepted input values
Option for input parameters to be flattened to string
See discussion in JENKINS-34101.
PR up at https://github.com/jenkinsci/pipeline-input-step-plugin/pull/16 - and turns out this is fixable in just pipeline-input-step.