WSO2 developer studio keeps removing square bracket from my json payload - wso2

Question

I'm using wso2 integration studio (version 7.2). I'm trying to post a JSON payload factory to an endpoint. This JSON message has a JSON array with a square bracket but wso2 keeps removing that before sending it out. The receiver API doesn't accept it in this format.
See my code below:
<api context="/test" name="testAPi" xmlns="http://ws.apache.org/ns/synapse">
<resource methods="POST">
<inSequence>
<property name="setCharacterEncoding" value="false" scope="axis2"/>
<payloadFactory media-type="json">
<format>{"field1": 5,"field2": 2022,"field3": "22","contacts": [{"id":1014,"surname": "N"}],"applicants": [{"nameId": 111111}]}
</format>
<args/>
</payloadFactory>
<header name="Content-Type" scope="transport" value="application/json"/>
<property name="contentType" scope="axis2" type="STRING" value="application/json"/>
<log level="full"/>
<header name="Accept" scope="transport" value="*/*"/>
<property name="messageType" scope="axis2" type="STRING" value="application/json"/>
<header name="Connection" scope="transport" value="keep-alive"/>
<header name="Accept-Encoding" scope="transport" value="gzip, deflate, br"/>
<property name="DISABLE_CHUNKING" scope="axis2" type="STRING" value="true"/>
<log level="full"/>
<call blocking="true">
<endpoint key="tcpmon"/>
</call>
<log level="full"/>
</inSequence>
<outSequence/>
<faultSequence/>
</resource>
I used the tcpmon to see what is sending out. and I can see wso2 just remove the [] from the message and send it out.
Do you know how can I fix this?