I wrote a whole epic here about my attempts to figure out what was going on, since it made no sense and I can't reproduce it. But I just noticed something truly weird:
WorkflowScript: 43: Unknown conditional error. Valid conditionals are: allOf, anyOf, branch, changelog, changeset, environment, expression, not @ line 43, column 21.
Specifically, Unknown conditional error - that means something went wrong, in theory, with parsing environment as a String literal, which...yeah. Never actually seen that, and it should have thrown up another error for that.
Is there any way you can get me the entire Jenkinsfile? Something somewhere is really broken with parsing your Jenkinsfile and I can't reproduce it just from your example.