In https://docs.openshift.com/container-platform/4.17/post_installation_configuration/configuring-private-cluster.html#private-clusters-setting-api-private_configuring-private-cluster , we have two steps in the "Procedure":
1. In the web portal or console for your cloud provider, take the following actions:
2. AWS clusters: Remove the external load balancers:

but actually step 2 "AWS clusters: Remove the external load balancers" should be placed in the first step, or else the CPMS couldn't be updated due to missing the External LB. 

# oc edit controlplanemachineset -n openshift-machine-api
controlplanemachineset.machine.openshift.io/cluster edited

# oc logs -f machine-api-controllers-7bdc49c5dd-f2npw -c machine-controller
...
E1125 07:01:46.152010       1 actuator.go:72] gpei-1125a-hq7cw-master-0 error: gpei-1125a-hq7cw-master-0: reconciler failed to Delete machine: failed to remove instance from load balancers: LoadBalancerNotFound: Load balancers '[gpei-1125a-hq7cw-ext]' not found
	status code: 400, request id: 8157ed26-92b7-47b4-9c68-059ac45039a8
E1125 07:01:46.152049       1 controller.go:274] gpei-1125a-hq7cw-master-0: failed to delete machine: gpei-1125a-hq7cw-master-0: reconciler failed to Delete machine: failed to remove instance from load balancers: LoadBalancerNotFound: Load balancers '[gpei-1125a-hq7cw-ext]' not found
	status code: 400, request id: 8157ed26-92b7-47b4-9c68-059ac45039a8
E1125 07:01:46.152107       1 controller.go:316] "msg"="Reconciler error" "error"="gpei-1125a-hq7cw-master-0: reconciler failed to Delete machine: failed to remove instance from load balancers: LoadBalancerNotFound: Load balancers '[gpei-1125a-hq7cw-ext]' not found\n\tstatus code: 400, request id: 8157ed26-92b7-47b4-9c68-059ac45039a8" "controller"="machine-controller" "name"="gpei-1125a-hq7cw-master-0" "namespace"="openshift-machine-api" "object"={"name":"gpei-1125a-hq7cw-master-0","namespace":"openshift-machine-api"} "reconcileID"="d75e1dd9-90b7-4d7e-b95c-279480fb924e"
I1125 07:01:46.152169       1 recorder.go:104] "msg"="gpei-1125a-hq7cw-master-0: reconciler failed to Delete machine: failed to remove instance from load balancers: LoadBalancerNotFound: Load balancers '[gpei-1125a-hq7cw-ext]' not found\n\tstatus code: 400, request id: 8157ed26-92b7-47b4-9c68-059ac45039a8" "logger"="events" "object"={"kind":"Machine","namespace":"openshift-machine-api","name":"gpei-1125a-hq7cw-master-0"

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