IIB - How to pass UserContext in a SOAPAsyncRequest node

I'm trying to pass the user context from a SOAPAsyncRequest node to a SOAPAsyncResponse node by overriding the local environment like described in the IBM documentation here and here.

Here is a part of my flow:

In Map to N compute node (Compute Mode is set on LocalEnvironment and Message):

SET OutputLocalEnvironment = InputLocalEnvironment;
SET OutputLocalEnvironment.Variables.RequestId = InputLocalEnvironment.Destination.HTTP.RequestIdentifier;
SET OutputLocalEnvironment.Variables.Language = InputRoot.HTTPInputHeader."Accept-Language";
SET OutputLocalEnvironment.Destination.SOAP.Request.UserContext = CAST(ASBITSTREAM(OutputLocalEnvironment.Variables) as BLOB);

In SetRequestId compute node:

DECLARE UserContext BLOB;
SET UserContext = InputLocalEnvironment.Destination.SOAP.Response.UserContext;

Debug value of the LocalEnvironment after the Map to N node:

LocalEnvironment
    Destination
        HTTP
            RequestIdentifier:BLOB:[B@54d21920
        RouterList
            DestinationData
                labelName:CHARACTER:GET
        SOAP
            Request
                UserContext:BLOB:[B@3d098be1

Debug value after the SOAP Async Response N node:

LocalEnvironment
    SOAP
        Response
            MessageCorrelId:BLOB:[B@a075817
    Destination
        RouterList
            DestinationData
                labelName:CHARACTER:findXYZ

Though the LocalEnvironment is correctly set after the Map to N node (see debug above), the value of UserContext is always null.

edit: Except for the UserContext problem, my flow is working properly. To answer the first comment, yes, the unique identifiers are correctly set.

I didn't succeed to pass the Variables tree structure but I found a workaround by concatenating the values and storing it into a BLOB. Neither it is the cleanest way to resolve it, nor is this solution easy to implement when dealing with more than two objects.

In Map To D Compute Node:

SET ConcatenatedContext = 'YourFirstString' || ';' || 'YourSecondString';
SET OutputLocalEnvironment.Destination.SOAP.Request.UserContext = CAST(ConcatenatedContext AS BLOB CCSID 1208);

In SetRequestId Compute Node:

DECLARE RawValue CHARACTER;
SET RawValue = CAST(InputLocalEnvironment.SOAP.Response.UserContext AS CHARACTER CCSID 1208);
SET FirstString = SUBSTRING(RawValue BEFORE ';');
SET SecondString = SUBSTRING(RawValue AFTER ';');