Verified on 4.2.0-0.nightly-2019-08-28-083236
Degraded reason is bubbled up to co/machine-config now


oc describe co/machine-config

cat broken2.yaml 
apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  labels:
    machineconfiguration.openshift.io/role: worker
  name: 50-examplecorp-chrony
spec:
  config:
    ignition:
      version: 9.2.0
    storage:
      files:
      - contents:
          source: data:text/plain;charset=utf;base64,c2VydmVyIGZvby5leGFtcGxlLm5ldCBtYXhkZWxheSAwLjQgb2ZmbGluZQpzZXJ2ZXIgYmFyLmV4YW1wbGUubmV0IG1heGRlbGF5IDAuNCBvZmZsaW5lCnNlcnZlciBiYXouZXhhbXBsZS5uZXQgbWF4ZGVsYXkgMC40IG9mZmxpbmUK
        filesystem: root
        mode: 0644
        path: /etc/ai

$ oc apply -f broken2.yaml
$ oc get machineconfigpool
NAME     CONFIG                                             UPDATED   UPDATING   DEGRADED
master   rendered-master-7624f2f26c9ffe65084229eaf38b7ad6   True      False      False
worker   rendered-worker-23e9267d9be51e29fa9370de4d79ca59   True      False      True

$ oc describe co/machine-config
Name:         machine-config
Namespace:    
Labels:       <none>
Annotations:  <none>
API Version:  config.openshift.io/v1
Kind:         ClusterOperator
Metadata:
  Creation Timestamp:  2019-08-29T17:05:41Z
  Generation:          1
  Resource Version:    126568
  Self Link:           /apis/config.openshift.io/v1/clusteroperators/machine-config
  UID:                 3affdf32-ca7f-11e9-9a80-0277603d43aa
Spec:
Status:
  Conditions:
    Last Transition Time:  2019-08-29T17:07:05Z
    Message:               Cluster has deployed 4.2.0-0.nightly-2019-08-28-083236
    Status:                True
    Type:                  Available
    Last Transition Time:  2019-08-29T17:07:06Z
    Message:               Cluster version is 4.2.0-0.nightly-2019-08-28-083236
    Status:                False
    Type:                  Progressing
    Last Transition Time:  2019-08-29T17:05:41Z
    Status:                False
    Type:                  Degraded
    Last Transition Time:  2019-08-29T17:07:05Z
    Reason:                AsExpected
    Status:                True
    Type:                  Upgradeable
  Extension:
    Master:  all 3 nodes are at latest configuration rendered-master-7624f2f26c9ffe65084229eaf38b7ad6
    Worker:  pool is degraded because rendering fails with "": "Failed to render configuration for pool worker: invalid Ignition config found: error: incorrect config version (too new)" 
  Related Objects:
    Group:     
    Name:      openshift-machine-config-operator
    Resource:  namespaces
    Group:     machineconfiguration.openshift.io
    Name:      master
    Resource:  machineconfigpools
    Group:     machineconfiguration.openshift.io
    Name:      worker
    Resource:  machineconfigpools
    Group:     machineconfiguration.openshift.io
    Name:      machine-config-controller
    Resource:  controllerconfigs
  Versions:
    Name:     operator
    Version:  4.2.0-0.nightly-2019-08-28-083236
Events:       <none>

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:2922