When monitoring a Oracle RAC one node instance, the RAC agent and node agents are created, the user can check on the agent status, but on the Global View, only the node agent is shown.
Access the alarm configuration to configure the email setting, add the email receiver for the "DBO - Invalid Materialized Views" alarm, trigger the alarm, the user will find they cannot receive this alarm email. Checking the alarm description, the user finds that the description shows the agent is a RAC agent, so this means this alarm is for the RAC agent, not the node agent.
Then, one the node agent alarm setting is applied to the RAC node, trigger the alarm again, it will receive the email.