Unread always 0 for channels in iFrame event unread-changed-by-subscription

Context:

I am recreating a chat interface around the embedded iFrame. So I am heavily using the rest API and iFrame events. This specific issue prevent my users from being notified in a real time way of a new message in a channel.

Description:

On receive the iFrame event unread-changed-by-subscription, if the property "t" equal "c", meaning that the event received is about a channel, unread property will always be 0.
Note that unread is set correctly if the property "t" equal "d", for direct messages.

=> So it seems that the unread property from unread-changed-by-subscription event is broken for channels but works for direct messages.

Steps to reproduce:

1. Create a channel (Not a direct message) with user A and B
2. Connect with user A, send a message to the channel created before
3. Connect with user B, see the event unread-changed-by-subscription coming in but unread is still 0.

Documentation: https://developer.rocket.chat/guides/developer/iframe-integration/iframe-events

Expected behavior:

I expect unread property to be set correctly on receive unread-changed-by-subscription for channels.

Actual behavior:

The unread property in the event data we receive on unread-changed-by-subscription remains at 0 at all times.

Server Setup Information:

• Version of Rocket.Chat Server: 3.16.1 and 3.16.3
• Operating System: Manjaro
• Deployment Method: docker-compose
• Number of Running Instances: 1
• Database Migration: 226
• NodeJS Version: 12.22.1
• MongoDB Version: 4.0.25

Client Setup Information

• Desktop App or Browser Version: 87.0.4280.88
• Operating System: Manjaro

Thank you

Reproduce the issue with a channel and a direct message using the documented unread-changed-by-subscription iFrame event. Compare the event data for channel and direct-message subscriptions, then trace where the unread value is produced. Done means channel events report the correct unread count, matching the expected behavior described in the issue.