Welcome to the Splat alpha!

After poking around, I believe the issues arises from this section of code which filters out recipients based on whether the change involves only updates for target reviewers: (https://github.com/reviewboard/reviewboard/blob/master/reviewboard/notifications/email/message.py#L507)

If this is the case, then owners of the review_request shouldn't experience the separate threading, this is because the build_recipients function will always include the user of the review_request. (https://github.com/reviewboard/reviewboard/blob/master/reviewboard/notifications/email/utils.py#L48)

Adding the reference header doesn't seem to be enough, since once the reviewers are added and the review_request draft is published, the message_id attached to the original review_request is modified (https://github.com/reviewboard/reviewboard/blob/master/reviewboard/notifications/email/signal_handlers.py#L261)
Solutions might involve keeping track of all previous message_ids and adding those to the Headers["References"] field, but that might be too much, otherwise removal of the filtering will solve it (but that might cause too much noise)

Nvm, I think this was an issue with internal stuff, this can be closed MY BAD

Fixed in 7.0.2

Ack, this got lost in the noise. The solution is to fix the media URL in /admin/settings/general/