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:

def a = b = c = 1 

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:

@Test public void assignmentExprsReturnRhs() throws Exception {
    WorkflowJob p = jenkins.createProject(WorkflowJob.class);
    p.setDefinition(new CpsFlowDefinition(
            "def a = b = c = 1\n" +
            "println([a, b, c].toString())", true));
    WorkflowRun b = jenkins.buildAndAssertSuccess(p);
    jenkins.assertLogContains("[1, 1, 1]", b); // Actual output today is "[null, null, 1]".
} 

I think the relevant code is AssignmentBlock in groovy-cps. In particular, I think that assignAndDone needs to create a new continuation to pass to lhs.set that calls k.receive(rhs) internally.