I am having the same problem. I created both a Server-to-Server app and a Legacy webhook-only app.
They both have the “in-meeting chat message received” event activated.
In addition to that event, I have the meeting ended event selected for both apps.
Everything is configured correctly and I receive data only for the “meeting ended” event on the Legacy webhook-only app.
There is a bug mentioned in this thread.
Support has also told me I have DLP enabled.
Is there a difference between the “in-meeting chat message received” event and the meeting.chat_message_sent webhook endpoint?
Is there an update on when this bug will be fixed?