Graph API - BadRequest on sending channel messages with #mention - asp.net-web-api

Question

We had some code that has been working for the past 10 months (since it was developed) and just stopped working this afternoon. It's a WebAPI code to send a channel message mentioning the bot and a user, which now is returning "Bad Request. Invalid request body was sent."
If the "Mentions" property is not provided, the call works, and the message is sent without the #mentions. So, I wonder if there was a breaking change in this API that's now expecting a different format for the "Mentions" property.
It's quite simple to reproduce by following the example code found in the Microsoft Graph documentation.
I'm posting here in the hope some fellow dev spots something obvious or is aware of an alternative way of using the API that it might stop complaining, as Microsoft takes forever to reply.
Here's the code we have that can lead me to discover the issue:
private async Task SendMentionToTheBotAsync(GraphServiceClient onBehalfOfClient, string userName, string teamId, string channelId)
{
var supportAgentUser = await onBehalfOfClient.Me.Request().GetAsync();
var chatMessage = new ChatMessage
{
Body = new ItemBody
{
ContentType = BodyType.Html,
Content = $"<at id=\"0\">{Configuration["BotName"]}</at>: This is the start of the conversation between {userName} and <at id=\"1\">{supportAgentUser.DisplayName}</at>."
},
Mentions = new List<ChatMessageMention>
{
new ChatMessageMention
{
Id = 0,
MentionText = Configuration["BotName"],
Mentioned = new IdentitySet
{
Application = new Identity
{
DisplayName = Configuration["BotName"],
Id = Configuration["BotAppId"],
AdditionalData = new Dictionary<string,object>
{
{
"applicationIdentityType", "bot"
}
}
}
}
},
new ChatMessageMention
{
Id = 1,
MentionText = supportAgentUser.DisplayName,
Mentioned = new IdentitySet
{
User = new Identity
{
DisplayName = supportAgentUser.DisplayName,
Id = supportAgentUser.Id,
AdditionalData = new Dictionary<string,object>
{
{
"userIdentityType", "aadUser"
}
}
}
}
}
}
};
await onBehalfOfClient.Teams[teamId].Channels[channelId].Messages
.Request()
.AddAsync(chatMessage);
}

Answer 1

Microsoft Support responded with :
"Thank you for contacting Microsoft Support.
I understand the issue is related to the post messages to Teams. Based on the screenshot, it seems you are using mention to a channel. It's possible that you are using key "conversationIdentityType#odata.type" in your request.
Could you please try to remove "conversationIdentityType#odata.type" key from the request body and try again. It should work. It is because deployment is on the way in the Asia region. Once it's 100% rolled out, this key WILL NOT be entertained in the request."
Removed the key and it worked for me.

Answer 2

Paulo,
Unfortunately i am not a programmer. I am using Graph calls in a Microsoft 365 Power Automate workflow. I have an app that i use to get the Authorisation Bearer token and then post to Teams messages using a graph HTTP action.
Here is the syntax of the HTTP ( purple items are variables if u r not familiar with Flow )
click to view image of Power Automate workflow HTTP action