Web Messaging participant attributes missing in Architect flow

We’re trying to access participant attributes set via the Web Messaging API in an Architect Inbound Message flow. The attributes show up fine in the UI, but using {{conversation.participants[0].attributes}} returns an empty object. We need this for WEM adherence tracking. Here’s the JSON mapping we’re using:

{
 "type": "SET",
 "target": "user:custom:msg_attr",
 "value": "{{conversation.participants[0].attributes}}"
}

It just stays blank. What’s the right path?