Issue with Teams 'Add a User to Group Chat' API call?

I am getting a very strange error when trying to add a new user to an existing group chat using a GraphQL call. I have looked through the documentation, asked AI, and even contacted Microsoft support and no one can seem to explain why this API call is failing. 

Below is the API call that I am making. I can find no reason that I should be receiving the error message '#microsoft.graph.aadUserConversationMember'

To get the basic questions out of the way....

• Yes, the account making the API call DOES have permission to add users to the channel (and can do so using the frontend as normal.)
  Yes, the thread in question is a group chat, so adding members to that chat should be a valid command. 
• Yes, ALL members in the group chat currently have the type '#microsoft.graph.aadUserConversationMember', so it is definitely a valid type for users in this channel. 
• Yes, I have tried both the beta and stable channel; each gives the same error message.
• Yes, the invited user is internal to our organization, and is a valid target for the invite (invites work through frontend as well.)
• No, I cannot use the 'add member to channel' endpoint, because the chat is a group chat, not a channel. 

I can only assume, at this point, that the error message is a red herring and there's something else wrong with my API call?Why am I receiving this error message when trying to add a member to a pre-existing group chat? Thanks in advance for any assistance.

POST to URL:
https://graph.microsoft.com/beta/chats/[[THREAD_ID]]@thread.v2/members

BODY:
{

"@odXXX.type": "#microsoft.graph.aadUserConversationMember",

"roles": [],

"email address removed for privacy reasons": "https://graph.microsoft.com/v1.0/users/[[USER_ID]]"

}

ERROR MESSAGE:
{"error":{"code":"BadRequest","message":"The provided '#microsoft.graph.aadUserConversationMember' for 'odXXX.type' is not valid for this operation.","innerError":{"date":"2026-01-29T18:10:32","request-id":"<PII:moderator removed>","client-request-id":"<PII:moderator removed>"}}}