Just wanted to share some of the research I did.
We installed a 2 node NBU cluster with 18.104.22.168 and SFHA 5.1 SP2. After installation the master server would always show DOWN in activity monitor. I confirmed on the forum and tech support but they just said that it is normal but no one could help me in getting rid of the RED thing which looks abnormal to anyone.
The trick was in bpclusterutil to remove the services which are not required for a master to function.
In my case I observed that Device manager and KMS service were going down after we bring up the cluster.
Revomed these services from registry -
First DOWN the NBU cluster resource.
Edit registry and go to
Remove the services which show up in Event viewer that they go down.
Bring up the NBU resource,
I hope it helps people.