As described in https://stackoverflow.com/questions/61420959/jenkins-interpretation-of-multiple-object-declarations-on-one-line, CPS-transformed assignment expressions evaluate to null instead of the value of the RHS of the expression. Here is an example:
In a Pipeline, after executing that expression a == null, b == null, and c == 1.
In regular Groovy, after executing that expression, a, b, and c, are all equal to 1.
Here is a (failing) workflow-cps test case (for CpsFlowDefinition2Test) that demonstrates the bug: