Java SDK: Accessing Web Messaging participant attributes in Architect Inbound Message flow

Trying to understand why my Spring Boot service cannot retrieve custom participant attributes set via Web Messaging when processing an Inbound Message flow in Architect.

I am using the Java SDK to push attributes to a conversation, but they do not appear in the Architect data action payload. The documentation states: “Participant attributes are available in the flow context if the interaction type supports them.”

Here is my Java code using the SDK:

ConversationApi convApi = new ConversationApi(apiClient);
ConversationParticipantAttributeSet attrSet = new ConversationParticipantAttributeSet();
attrSet.setCustomAttributes(new LinkedHashMap<>());
attrSet.getCustomAttributes().put("source", "webchat");
convApi.conversationsParticipantsAttributesPut(conversationId, participantId, attrSet, null, null);

The API returns 204 No Content, indicating success. However, in Architect, when I map the data action, the JSON payload looks like this:

payload:
 participant_attributes:
 custom:
 # source is missing here

Is there a specific API endpoint or SDK method I should use to ensure these attributes persist for Inbound Message interactions? The Web Messaging channel seems to handle attributes differently than Voice or Chat.