API Monitoring cannot catch Raise Fault fault code

Google Cloud Apigee
1

My alert is based on steps.raisefault.RaiseFault fault code. When it is thrown then API Monitoring catch it and send alert. Hence, I need to throw a RaiseFault in APIGEE error flow along with happy flow. However, I do not know why recently API Monitoring cannot catch RaiseFault fault code when RaiseFault is thrown in anymore. I used API Monitoring/Investigate to check then API Monitoring considers RaiseFault as messaging.adaptors.http.flow.ErrorResponseCode fault code now. As a consequence, there is no alert when target server returns error. Is it a APIGEE bug? Does anyone know the reason ?. Thanks alot.

<DefaultFaultRule name="xxx">
        <Step>
            <Name>Raise-Fault-example</Name>
            <Condition>abc = "true"</Condition>
        </Step>
</DefaultFaultRule>

RaiseFault code:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<RaiseFault async="false" continueOnError="false" enabled="true" name="Raise-Fault-1">
    <DisplayName>Raise Fault-1</DisplayName>
    <Properties/>
    <FaultResponse>
        <Set>
            <Headers/>
            <Payload contentType="text/plain"/>
            <StatusCode>500</StatusCode>
            <ReasonPhrase>Server Error</ReasonPhrase>
        </Set>
    </FaultResponse>
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
</RaiseFault>

@Dino-at-Google Do you know this strange behavior?