In JENKINS-52038 it was introduced that parameter values not defined in the choice parameter get rejected.

Same thing has been done to the Parameterized Trigger Plugin some time ago. But for this plugin there is a switch (hudson.plugins.parameterizedtrigger.ProjectSpecificParametersActionFactory.compatibility_mode=true) to allow undefined parameter values.

There should also be a switch like that for the Pipeline Build Step Plugin to let the user decide whether he thinks it is safe to accept undefined values or not.