RHACM breaks enableUserAlertmanagerConfig

Description of problem:

My customer has seen the following bug:

When you enable ```enableUserAlertmanagerConfig``` according to documentation https://docs.openshift.com/container-platform/4.11/monitoring/enabling-monitoring-for-user-defined-projects.html

And then you add the cluster to RHACM, it will remove this configuration setting completely from configmap ```cluster-monitoring-config``` in namespace ```openshift-monitoring``` and restarts the pods from ```statefulset.apps/prometheus-k8s```.

We tested this on many different clusters.

For this setting to stay active you will need to enable it after adding the cluster to RHACM.

Version-Release number of selected component (if applicable):

Will confirm this

How reproducible:

100%

Steps to Reproduce:

See above

Expected results:

Additional info: