Description of problem:

https://docs.openshift.com/container-platform/4.13/machine_management/deploying-machine-health-checks.html#machine-health-checks-about_deploying-machine-health-checks

The target machineset of machine health checks are incorrect. It has the opposite meaning. The machine health checks can not checks control plane.

NOTE
You can only apply a machine health check to control plane machines on clusters that use control plane machine sets. 

Until 4.11, The satet is below.

https://docs.openshift.com/container-platform/4.11/machine_management/deploying-machine-health-checks.html#machine-health-checks-about_deploying-machine-health-checks

You cannot apply a machine health check to a machine with the master role. 

Simply  we can change machine with the master role to control plane machines.