I did a quick test on Pega 7.1.9 following below steps:
1. Deleted the Agent schedule from SysAdmin --> Agent schedule ( had three instance for same agent)
2. Went to SMA to check the status of Agent, it was shown running and isAlive test results in true.
I believe, this is due to the fact that we deleted only Agent schedule for that node , therefore, agent will keep showing as running ( we did not delete the agent) as it needs to cater other agent schedule for this agent.