The following document provides an example how to handle native NonCPS methods like toString.

https://wiki.jenkins.io/display/JENKINS/Pipeline+CPS+method+mismatches#PipelineCPSmethodmismatches-Overridesofnon-CPS-transformedmethods

class Test {
  @Override
  public String toString() {
    return "Test"
  }
}
def builder = new StringBuilder()
builder.append(new Test())
echo(builder.toString()) 

But this example is way to trivial, as toString in 99,99% returns variable information usually provided by a couple of handy methods of the class, like in this case with CPS getFullName method

class Test {
    String name

    Test(String name) {
        this.name = name
    }

    String getFullName() {
        return 'some prefix ' + name
    }

    @NonCPS
    @Override
    public String toString() {
        return this.fullName
    }

}

def builder = new StringBuilder()
builder.append(new Test('foobar'))
echo(builder.toString())

Previously this code worked as it doesn't call any steps as the document suggest, but now it started to raise 

java.lang.IllegalStateException: expected to call java.lang.StringBuilder.append but wound up catching Test.getFullName; see: https://jenkins.io/redirect/pipeline-cps-method-mismatches

Obviously it's possible to put in @NonCPS for getFullName but that simply means that almost all methods by default must be @NonCPS, and only those that call steps must be CPS ones.

Can we have this 
'org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService.handleMismatch'
properly fixed as there's actually no mismatch, as even explicit toString

(new Test('foobar')).toString() 

doesn't help to work this around?