WSO2 Micro Integrator: Base64 SOAP Response to JSON Causes AxisFault "Error getting GZIP output stream"

I have a WSO2 Micro Integrator proxy that calls a WSDL endpoint. The response is a SOAP envelope containing a list of documents, each with a contents field that holds a Base64-encoded file. I extract this Base64 content, convert the response to JSON, and send it to a Spring Boot service. However, I encounter the following error when sending the request:

[2025-03-28 23:51:41,696] ERROR {Axis2Sender} - {proxy:CallbackDecision} Unexpected error during sending message out. Error getting GZIP output stream org.apache.axis2.AxisFault: Error getting GZIP output stream

Observations:

When I intercept the HTTP request in Spring Boot, the body appears as bytes with weird characters instead of a valid JSON string.

org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Illegal character ((CTRL-CHAR, code 31)): only regular white space (\r, \n, \t) is allowed between tokens

If I hardcode the payload in WSO2 (instead of processing the SOAP response), the request to the backend service works fine (only if I disable the wsdl endpoint call or if response does not contains base64 or the response if a fault (small)).

                 <sequence>
                        <property name="uri.var.loanId" scope="default" type="STRING"
                            expression="//mi:numDemande"
                            action="set"
                            xmlns:mi="http://mi.sofac.ma" />
                        <log category="INFO" level="full">
                            <property name="loanID" expression="$ctx:uri.var.loanId" />
                        </log>
                        <payloadFactory media-type="xml" template-type="default">

                            <format><docListReception
                                    xmlns="******">
                                    <offerId xmlns="">309264</offerId>
                                    <dealerLogin xmlns="">SAMG1001</dealerLogin>
                                    <documentType xmlns=""></documentType>
                                    <documentNature xmlns=""></documentNature>
                                    <documentCode xmlns="">D0053</documentCode>
                                </docListReception></format>
                            <args>
                            </args>
                        </payloadFactory>
                        <header name="Action"
                            value=" " />


                        <call>
                            <endpoint key="DocListReception" />

                        </call>

                        <property name="base64" action="set" type="STRING"
                            expression="//fileContents"></property>
                        <log category="INFO" level="simple">
                            <property name="base64" expression="$ctx:base64" />
                        </log>
                        <payloadFactory media-type="json" template-type="default">
                            <format> {"docsToUpload":[{"fileNature":"A0023","fileType":"CONTRACT",
                                "fileCode":"D0051","fileContents":"$1", "longLabelDocument":"Proces
                                verbal de reception SFP"}]} </format>
                            <args>
                                <arg evaluator="xml" expression="$ctx:base64" />
                            </args>
                        </payloadFactory>
                        <call>
                            <endpoint key="samStagingUploadFD" />

                        </call>
                    </sequence>

I pretty sure that the problem is not while converting, but while sending.

can I someone help me ^^