Solved: Thanks to below answer from S.Richmond. I needed to unset all stored maps of the groovy.json.internal.LazyMap type whi
The way pipeline plugin has been implemented has quite serious implications for non-trivial Groovy code. This link explains how to avoid possible problems: https://github.com/jenkinsci/pipeline-plugin/blob/master/TUTORIAL.md#serializing-local-variables
In your specific case I'd consider adding @NonCPS annotation to slurpJSON and returning map-of-maps instead of JSON object. Not only the code looks cleaner, but it's also more efficient, especially if that JSON is complex.